terminalShop/api/handlers/cart.go

package handlers

import (
	"context"
	"encoding/json"
	"fmt"
	"net/http"
	"strconv"
	"time"

	"github.com/go-chi/chi/v5"
	"github.com/stripe/stripe-go/v78"
	"github.com/stripe/stripe-go/v78/paymentintent"
	"github.com/stripe/stripe-go/v78/paymentmethod"
	"gorm.io/gorm"

	"go.opentelemetry.io/otel"

	"terminalShop/api/middleware"
	"terminalShop/pkg/audit"
	"terminalShop/pkg/database"
	"terminalShop/pkg/models"
	"terminalShop/pkg/notify"
	"terminalShop/pkg/utils"
)

type CartHandler struct {
	appURL        string
	maxOrderCents int
}

// maxAuthIssuances caps how many times a single ordes's PaymentIntent can be
// pushed into requires_action; one initial challende plus up to three retries
// before we tell the customer to use a different card. Counted via existing
// audit rows (audit.EventOrderRequiresAction) so the cap stays migration-free.
const maxAuthIssuances = 4

// NewCartHandler creates a new cart handler. Stripe credentials are wired
// once at startup via stripe.Key in api/main.go.
func NewCartHandler(appURL string, maxOrderCents int) *CartHandler {
	return &CartHandler{appURL: appURL, maxOrderCents: maxOrderCents}
}

// getOrCreateCart finds the users cart or creates one.
func getOrCreateCart(ctx context.Context, userID uint) (*models.Cart, error) {
	db := database.GetDB().WithContext(ctx)
	var cart models.Cart
	err := db.Where("user_id = ?", userID).First(&cart).Error
	if err != nil {
		cart = models.Cart{UserID: userID}
		if createErr := db.Create(&cart).Error; createErr != nil {
			return nil, fmt.Errorf("failed to create cart: %w", createErr)
		}
	}
	return &cart, nil
}

func cartResponse(ctx context.Context, cart *models.Cart) map[string]any {
	db := database.GetDB().WithContext(ctx)

	var items []models.CartItem
	db.Where("cart_id = ? AND quantity > 0", cart.ID).Preload("Coffee").Find(&items)

	subtotal := 0
	itemsData := make([]map[string]any, 0, len(items))
	for _, item := range items {
		lineTotal := item.Coffee.Price * item.Quantity
		subtotal += lineTotal
		itemsData = append(itemsData, map[string]any{
			"id":        item.ID,
			"coffee_id": item.CoffeeID,
			"quantity":  item.Quantity,
			"subtotal":  lineTotal, // already in cents
			"coffee":    item.Coffee,
		})
	}

	return map[string]any{
		"items":            itemsData,
		"subtotal":         subtotal,
		"address_id":       cart.AddressID,
		"card_id":          cart.CardID,
		"shipping_cost":    cart.ShippingCost,
		"shipping_service": cart.ShippingService,
	}
}

func (h *CartHandler) GetCart(w http.ResponseWriter, r *http.Request) {
	userID := middleware.UserIDFromContext(r.Context())

	cart, err := getOrCreateCart(r.Context(), userID)
	if err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "CART_ERROR", "failed to get cart", nil)
		return
	}

	utils.RespondSuccess(w, http.StatusOK, map[string]any{
		"cart": cartResponse(r.Context(), cart),
	})
}

type SetItemRequest struct {
	CoffeeID uint `json:"coffee_id"`
	Quantity int  `json:"quantity"`
}

func (h *CartHandler) SetItem(w http.ResponseWriter, r *http.Request) {
	db := database.GetDB().WithContext(r.Context())
	userID := middleware.UserIDFromContext(r.Context())

	var req SetItemRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_JSON", "invalid request body", nil)
		return
	}

	if req.CoffeeID == 0 {
		utils.RespondError(w, http.StatusBadRequest, "VALIDATION_ERROR", "coffee_id is required", nil)
		return
	}

	var coffee models.Coffee
	if err := db.First(&coffee, req.CoffeeID).Error; err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_PRODUCT", "product not found", nil)
		return
	}

	cart, err := getOrCreateCart(r.Context(), userID)
	if err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "CART_ERROR", "failed to get cart", nil)
		return
	}

	if req.Quantity <= 0 {
		if err := db.Where("cart_id = ? AND coffee_id = ?", cart.ID, req.CoffeeID).Delete(&models.CartItem{}).Error; err != nil {
			utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to delete cart item", nil)
			return
		}
	} else {
		var item models.CartItem
		result := db.Where("cart_id = ? AND coffee_id = ?", cart.ID, req.CoffeeID).First(&item)
		if result.Error != nil {
			item = models.CartItem{
				CartID:   cart.ID,
				CoffeeID: req.CoffeeID,
				Quantity: req.Quantity,
			}
			if err := db.Create(&item).Error; err != nil {
				utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to create item", nil)
				return
			}
		} else {
			if err := db.Model(&item).Update("quantity", req.Quantity).Error; err != nil {
				utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to update quantity", nil)
				return
			}
		}
	}

	utils.RespondSuccess(w, http.StatusOK, map[string]any{
		"cart": cartResponse(r.Context(), cart),
	})
}

type SetAddressRequest struct {
	AddressID uint `json:"address_id"`
}

func (h *CartHandler) SetAddress(w http.ResponseWriter, r *http.Request) {
	db := database.GetDB().WithContext(r.Context())
	userID := middleware.UserIDFromContext(r.Context())

	var req SetAddressRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_JSON", "invalid request body", nil)
		return
	}

	var address models.Address
	if err := db.Where("id = ? AND user_id = ?", req.AddressID, userID).First(&address).Error; err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_ADDRESS", "address not found", nil)
		return
	}

	cart, err := getOrCreateCart(r.Context(), userID)
	if err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "CART_ERROR", "failed to get cart", nil)
		return
	}

	if err := db.Model(cart).Update("address_id", req.AddressID).Error; err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to set address", nil)
		return
	}

	utils.RespondSuccess(w, http.StatusOK, map[string]any{
		"message": "address set",
	})
}

type SetCardRequest struct {
	CardID uint `json:"card_id"`
}

func (h *CartHandler) SetCard(w http.ResponseWriter, r *http.Request) {
	db := database.GetDB().WithContext(r.Context())
	userID := middleware.UserIDFromContext(r.Context())

	var req SetCardRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_JSON", "invalid request body", nil)
		return
	}

	var card models.Card
	if err := db.Where("id = ? AND user_id = ?", req.CardID, userID).First(&card).Error; err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_CARD", "card not found", nil)
		return
	}

	if card.IsStorageExpired(time.Now()) {
		if err := expireStoredCard(db, &card); err != nil {
			utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to expire card", nil)
			return
		}
		respondCardStorageExpired(w)
		return
	}

	cart, err := getOrCreateCart(r.Context(), userID)
	if err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "CART_ERROR", "failed to get cart", nil)
		return
	}

	if err := db.Model(cart).Update("card_id", req.CardID).Error; err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to set card", nil)
		return
	}

	utils.RespondSuccess(w, http.StatusOK, map[string]any{
		"message": "card set",
	})
}

func (h *CartHandler) ClearCart(w http.ResponseWriter, r *http.Request) {
	db := database.GetDB().WithContext(r.Context())
	userID := middleware.UserIDFromContext(r.Context())

	cart, err := getOrCreateCart(r.Context(), userID)
	if err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "CART_ERROR", "failed to get cart", nil)
		return
	}

	if err := db.Where("cart_id = ?", cart.ID).Delete(&models.CartItem{}).Error; err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to delete cart item", nil)
		return
	}

	utils.RespondSuccess(w, http.StatusOK, map[string]any{
		"message": "cart cleared",
	})
}

func (h *CartHandler) ConvertCart(w http.ResponseWriter, r *http.Request) {
	db := database.GetDB().WithContext(r.Context())
	userID := middleware.UserIDFromContext(r.Context())

	ctx, span := otel.Tracer("api").Start(r.Context(), "cart.convert")
	defer span.End()
	r = r.WithContext(ctx)

	cart, err := getOrCreateCart(r.Context(), userID)
	if err != nil {
		middleware.RecordCartConversion("cart_lookup_failed")
		utils.RespondError(w, http.StatusInternalServerError, "CART_ERROR", "failed to get cart", nil)
		return
	}

	var items []models.CartItem
	db.Where("cart_id = ? AND quantity > 0", cart.ID).Preload("Coffee").Find(&items)

	if len(items) == 0 {
		middleware.RecordCartConversion("validation_empty_cart")
		utils.RespondError(w, http.StatusBadRequest, "VALIDATION_ERROR", "cart is empty", nil)
		return
	}

	if cart.AddressID == nil {
		middleware.RecordCartConversion("validation_missing_address")
		utils.RespondError(w, http.StatusBadRequest, "VALIDATION_ERROR", "shipping address is required", nil)
		return
	}

	if cart.CardID == nil {
		middleware.RecordCartConversion("validation_missing_card")
		utils.RespondError(w, http.StatusBadRequest, "VALIDATION_ERROR", "payment card is required", nil)
		return
	}

	var address models.Address
	if err := db.First(&address, *cart.AddressID).Error; err != nil {
		middleware.RecordCartConversion("validation_invalid_address")
		utils.RespondError(w, http.StatusBadRequest, "INVALID_ADDRESS", "shipping address not found", nil)
		return
	}

	var card models.Card
	if err := db.First(&card, *cart.CardID).Error; err != nil {
		middleware.RecordCartConversion("validation_invalid_card")
		utils.RespondError(w, http.StatusBadRequest, "INVALID_CARD", "payment card not found", nil)
		return
	}

	if card.IsStorageExpired(time.Now()) {
		middleware.RecordCartConversion("validation_card_expired")
		if err := expireStoredCard(db, &card); err != nil {
			utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to expire card", nil)
			return
		}
		respondCardStorageExpired(w)
		return
	}

	var user models.User
	if err := db.First(&user, userID).Error; err != nil {
		middleware.RecordCartConversion("user_not_found")
		utils.RespondError(w, http.StatusNotFound, "USER_NOT_FOUND", "user not found", nil)
		return
	}

	// Ensure the user has a Stripe customer (shared helper with cards handler).
	if err := ensureStripeCustomer(db, &user); err != nil {
		middleware.RecordCartConversion("stripe_customer_failed")
		utils.RespondError(w, http.StatusInternalServerError, "STRIPE_ERROR", "failed to create stripe customer", nil)
		return
	}

	// All cards are now saved as pm_ PaymentMethods. This branch handles any
	// legacy tok_ records that predate the server-side tokenization refactor.
	paymentMethodID := card.StripePaymentID
	if len(paymentMethodID) >= 3 && paymentMethodID[:3] == "tok" {
		pm, pmErr := paymentmethod.New(&stripe.PaymentMethodParams{
			Type: stripe.String("card"),
			Card: &stripe.PaymentMethodCardParams{
				Token: stripe.String(paymentMethodID),
			},
		})
		if pmErr != nil {
			utils.RespondError(w, http.StatusInternalServerError, "STRIPE_ERROR", "failed to create payment method", nil)
			return
		}
		if _, attachErr := paymentmethod.Attach(pm.ID, &stripe.PaymentMethodAttachParams{
			Customer: stripe.String(user.StripeCustomerID),
		}); attachErr != nil {
			utils.RespondError(w, http.StatusInternalServerError, "STRIPE_ERROR", "failed to attach payment method", nil)
			return
		}
		paymentMethodID = pm.ID
		card.StripePaymentID = pm.ID
		if err := db.Save(&card).Error; err != nil {
			utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to save card", nil)
			return
		}
	}

	var subtotal int
	var orderItems []models.OrderItem
	for _, item := range items {
		lineTotal := item.Coffee.Price * item.Quantity
		subtotal += lineTotal
		orderItems = append(orderItems, models.OrderItem{
			CoffeeID: item.CoffeeID,
			Name:     item.Coffee.Name,
			Price:    item.Coffee.Price,
			Quantity: item.Quantity,
		})
	}

	total := subtotal + cart.ShippingCost

	if h.maxOrderCents > 0 && total > h.maxOrderCents {
		middleware.RecordCartConversion("validation_over_limit")
		utils.RespondError(w, http.StatusBadRequest, "CART_OVER_LIMIT",
			fmt.Sprintf("order total must be at most $%.2f", float64(h.maxOrderCents)/100),
			map[string]any{
				"total_cents": total,
				"limit_cents": h.maxOrderCents,
			})
		return
	}

	if total < 50 {
		middleware.RecordCartConversion("validation_min_amount")
		utils.RespondError(w, http.StatusBadRequest, "VALIDATION_ERROR", "order total must be at least $0.50", nil)
		return
	}

	// Create the order record BEFORE charging so we always have something to
	// reconcile against if the server crashes mid-flight.
	order := models.Order{
		UserID:          user.ID,
		CardID:          card.ID,
		StripePaymentID: "",
		Status:          models.OrderStatusPending,
		Subtotal:        subtotal,
		ShippingCost:    cart.ShippingCost,
		Total:           total,
		ShippingName:    address.Name,
		ShippingStreet:  address.Street,
		ShippingStreet2: address.Street2,
		ShippingCity:    address.City,
		ShippingState:   address.State,
		ShippingZip:     address.Zip,
		ShippingCountry: address.Country,
		ShippingPhone:   address.Phone,
		Items:           orderItems,
	}

	if err := db.Create(&order).Error; err != nil {
		middleware.RecordCartConversion("db_create_order_failed")
		utils.RespondError(w, http.StatusInternalServerError, "DATABASE_ERROR", "failed to create order", nil)
		return
	}

	audit.OrderCreated(userID, order.ID, total)

	// Charge the card. The idempotency key is tied to the order ID so retries
	// never produce a double charge.
	piParams := &stripe.PaymentIntentParams{
		Amount:             stripe.Int64(int64(total)),
		Currency:           stripe.String(string(stripe.CurrencyUSD)),
		Customer:           stripe.String(user.StripeCustomerID),
		PaymentMethod:      stripe.String(paymentMethodID),
		OffSession:         stripe.Bool(true),
		PaymentMethodTypes: stripe.StringSlice([]string{"card"}),
		ReturnURL:          stripe.String(h.appURL + "/post-3ds"),
		Confirm:            stripe.Bool(true),
		Description:        stripe.String(fmt.Sprintf("terminal.shop order #%d", order.ID)),
		Shipping: &stripe.ShippingDetailsParams{
			Name: stripe.String(address.Name),
			Address: &stripe.AddressParams{
				Line1:      stripe.String(address.Street),
				Line2:      stripe.String(address.Street2),
				City:       stripe.String(address.City),
				State:      stripe.String(address.State),
				PostalCode: stripe.String(address.Zip),
				Country:    stripe.String(address.Country),
			},
		},
	}
	piParams.SetIdempotencyKey(fmt.Sprintf("order-%d", order.ID))
	piParams.Metadata = map[string]string{
		"order_id": fmt.Sprintf("%d", order.ID),
	}

	stripeStart := time.Now()
	pi, err := paymentintent.New(piParams)
	stripeDur := time.Since(stripeStart).Seconds()
	if err != nil {
		middleware.ObserveStripeRequest("payment_intent_create", "error", stripeDur)
		if stripeErr, ok := err.(*stripe.Error); ok {
			if stripeErr.Code == stripe.ErrorCodeAuthenticationRequired && stripeErr.PaymentIntent != nil {
				h.respondRequiresAction(w, &order, paymentMethodID, stripeErr.PaymentIntent)
				return
			}
			db.Model(&order).Update("status", models.OrderStatusFailed)
			switch stripeErr.Type {
			case stripe.ErrorTypeCard:
				middleware.RecordCartConversion("card_declined")
				audit.OrderFailed(userID, order.ID, total, stripeErr.Msg)
				utils.RespondError(w, http.StatusPaymentRequired, mapStripeCardErrCode(stripeErr.Code), stripeErr.Msg, nil)
			case stripe.ErrorTypeInvalidRequest:
				middleware.RecordCartConversion("stripe_invalid_request")
				audit.OrderFailed(userID, order.ID, total, stripeErr.Msg)
				utils.RespondError(w, http.StatusBadRequest, "INVALID_PAYMENT", stripeErr.Msg, nil)
			default:
				middleware.RecordCartConversion("stripe_error")
				audit.OrderFailed(userID, order.ID, total, stripeErr.Error())
				utils.RespondError(w, http.StatusInternalServerError, "PAYMENT_FAILED", "payment failed", nil)
			}
			return
		}
		db.Model(&order).Update("status", models.OrderStatusFailed)
		middleware.RecordCartConversion("stripe_error")
		audit.OrderFailed(userID, order.ID, total, err.Error())
		utils.RespondError(w, http.StatusInternalServerError, "PAYMENT_FAILED", "payment failed", nil)
		return
	}
	middleware.ObserveStripeRequest("payment_intent_create", "success", stripeDur)

	if pi.Status == stripe.PaymentIntentStatusRequiresAction {
		h.respondRequiresAction(w, &order, paymentMethodID, pi)
		return
	}

	// Commit payment ID and clear cart in a single transaction.
	txErr := db.Transaction(func(tx *gorm.DB) error {
		order.StripePaymentID = pi.ID
		order.Status = models.OrderStatusPaid
		if err := tx.Save(&order).Error; err != nil {
			return err
		}
		if err := tx.Where("cart_id = ?", cart.ID).Delete(&models.CartItem{}).Error; err != nil {
			return err
		}
		return tx.Model(cart).Updates(map[string]any{
			"address_id": nil,
			"card_id":    nil,
		}).Error
	})
	if txErr != nil {
		// Card was charged but we could not persist the outcome. This is the
		// most dangerous failure mode: the customer paid but has no order.
		// The reconciliation job will catch this within 5 minutes.
		// The audit log entry below must be treated as a critical alert.
		middleware.RecordCartConversion("payment_critical")
		audit.PaymentCritical(order.ID, pi.ID, txErr)
		utils.RespondError(w, http.StatusInternalServerError, "DATABASE_ERROR", "payment succeeded but failed to record order — please contact support", nil)
		return
	}

	audit.OrderPaid(userID, order.ID, total, pi.ID)
	middleware.RecordOrderCreated(string(order.Status))
	middleware.RecordCartConversion("success")
	middleware.ObserveOrderValueCents(total)

	if err := db.Preload("Items").First(&order, order.ID).Error; err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to preload items", nil)
		return
	}
	go notify.SlackOrderPaid(&order)

	utils.RespondSuccess(w, http.StatusOK, map[string]any{
		"order": order,
	})
}

// respondRequiresAction handles a PaymentIntent that needs 3DS auth. The order
// transitions to requires_action (see caveat #20 in sca-psd2-compliance.md);
// the payment_intent.succeeded webhook flips it to paid once the customer
// completes the bank-hosted challenge. The TUI polls /api/v1/orders/{id}/status.
func (h *CartHandler) respondRequiresAction(w http.ResponseWriter, order *models.Order, paymentMethodID string, pi *stripe.PaymentIntent) {
	db := database.GetDB()

	// After an off-session authentication_required decline, the PI sits in
	// status=requires_payment_method with payment_method detached. Re-confirm
	// on-session with the same PM so Stripe pushes the PI into requires_action
	// and populates next_action.redirect_to_url with the 3DS challenge URL.
	if pi.NextAction == nil || pi.NextAction.RedirectToURL == nil || pi.NextAction.RedirectToURL.URL == "" {
		confirmed, cerr := paymentintent.Confirm(pi.ID, &stripe.PaymentIntentConfirmParams{
			PaymentMethod: stripe.String(paymentMethodID),
			OffSession:    stripe.Bool(false),
			ReturnURL:     stripe.String(h.appURL + "/post-3ds"),
		})
		if cerr == nil && confirmed != nil {
			pi = confirmed
		}
	}

	if pi.NextAction == nil || pi.NextAction.RedirectToURL == nil || pi.NextAction.RedirectToURL.URL == "" {
		middleware.RecordCartConversion("sca_missing_next_action")
		db.Model(order).Update("status", models.OrderStatusFailed)
		audit.OrderFailed(order.UserID, order.ID, order.Total, "stripe returned requires_action without redirect url")
		utils.RespondError(w, http.StatusBadGateway, "PAYMENT_FAILED", "stripe did not return a 3ds url", nil)
		return
	}

	// Persist the paymentIntent id AND set status to requires_action so list
	// UIs can distinguish "charging" from "awaiting customer 3DS". The
	// payment_intent.payment_failed webhook from the initial off-session
	// decline often arrives before this response and flips the order to
	// failed - override it back so the 3DS-in-flight order shows the
	// correct state
	if err := db.Model(order).Updates(map[string]any{
		"stripe_payment_id": pi.ID,
		"status":            models.OrderStatusRequiresAction,
	}).Error; err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "DATABASE_ERROR", "failed to save payment id", nil)
		return
	}

	token, err := storeRedirect(pi.NextAction.RedirectToURL.URL, models.RedirectPurposeCheckout3DS)
	if err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to allocate redirect token", nil)
		return
	}

	middleware.RecordCartConversion("requires_action")
	audit.OrderRequiresAction(order.UserID, order.ID, pi.ID)

	utils.RespondSuccess(w, http.StatusAccepted, map[string]any{
		"order_id":          order.ID,
		"payment_intent_id": pi.ID,
		"status":            "requires_action",
		"redirect_token":    token,
		"redirect_url":      fmt.Sprintf("%s/pay/%s", h.appURL, token),
	})
}

func mapStripeCardErrCode(code stripe.ErrorCode) string {
	switch code {
	case stripe.ErrorCodeAuthenticationRequired:
		return "AUTHENTICATION_REQUIRED"
	case "authentication_not_supported":
		return "AUTHENTICATION_NOT_SUPPORTED"
	case "card_declined", "":
		return "CARD_DECLINED"
	case "expired_card":
		return "CARD_EXPIRED"
	case "incorrect_cvc":
		return "CARD_CVC_FAILED"
	case "insufficient_funds":
		return "CARD_INSUFFICIENT_FUNDS"
	case "processing_error":
		return "CARD_PROCESSING_ERROR"
	}
	return "CARD_DECLINED"
}

//nolint:unused // wired into Convert cart later
func appURLForHandler(r *http.Request) string {
	scheme := "http"
	if r.TLS != nil || r.Header.Get("X-Forwarded-Proto") == "https" {
		scheme = "https"
	}
	host := r.Host
	if fh := r.Header.Get("X-Forwarded-Host"); fh != "" {
		host = fh
	}
	return fmt.Sprintf("%s://%s", scheme, host)
}

// RetryAuth re-Confirms an in-flight PaymentIntent that the customer failed
// or abandoned during the 3DS challenge, recovering the existing order rather
// than starting a new checkout. Bound to POST /api/v1/orders/{id}/retry-auth.
//
// Stripe's PI state machine bounces back to requires_payment_method after a
// failed or abandoned 3DS, so paymentintent.Confirm(pi.id, …) mints a fresh
// next_action.redirect_to_url — the same shape respondRequiresAction already
// handles. No idempotency key is set: Confirm mutates an existing PI and
// Stripe collapses by id (see sca-psd2-compliance.md caveat #5).
func (h *CartHandler) RetryAuth(w http.ResponseWriter, r *http.Request) {
	db := database.GetDB().WithContext(r.Context())
	userID := middleware.UserIDFromContext(r.Context())

	idStr := chi.URLParam(r, "id")
	id64, err := strconv.ParseUint(idStr, 10, 32)
	if err != nil {
		utils.RespondError(w, http.StatusBadRequest, "INVALID_ID", "invalid order id", nil)
		return
	}
	orderID := uint(id64)

	var order models.Order
	if err := db.Where("id = ? AND user_id = ?", orderID, userID).First(&order).Error; err != nil {
		utils.RespondError(w, http.StatusNotFound, "ORDER_NOT_FOUND", "order not found", nil)
		return
	}

	// Retryable source states:
	//	pending 		- initial attempt mid-flight (pre-PI confirm page)
	//	requires_action - customer abandoned or failed the 3DS challenge
	//	failed 			- webhook flipped after a non-recoverable error
	// terminal states (paid, shipped, delivered, cancelled, refunded) refuse.
	if order.Status != models.OrderStatusPending &&
		order.Status != models.OrderStatusRequiresAction &&
		order.Status != models.OrderStatusFailed {
		utils.RespondError(w, http.StatusConflict, "INVALID_STATE", "order is already finalized", nil)
		return
	}
	if order.StripePaymentID == "" {
		utils.RespondError(w, http.StatusConflict, "INVALID_STATE", "order has no payment intent to retry", nil)
		return
	}

	// Cap is best-effort: two concurrent retries at issuances=cap-1 will both
	// pass this check and both Confirm before either writes a new event row.
	// Per-user RateLimitByUser(10/min) on the route bounds the worst case.
	// SQLite-WAL has no SELECT ... FOR UPDATE, so a stricter guard would need
	// a serializing INSERT - not worth the cimplexity at current traffic.
	var issuances int64
	if err := db.Model(&models.OrderEvent{}).
		Where("order_id = ? AND type = ?", order.ID, audit.EventOrderRequiresAction).
		Count(&issuances).Error; err != nil {
		utils.RespondError(w, http.StatusInternalServerError, "DATABASE_ERROR", "failed to count auth attempts", nil)
		return
	}
	if issuances >= maxAuthIssuances {
		middleware.RecordCartConversion("retry_max_attempts")
		utils.RespondError(w, http.StatusTooManyRequests, "RETRY_LIMIT", "too many authentication attempts, please use a different card", nil)
		return
	}

	// scoped card lookup - match the initial-flow policy in ConvertCart so a
	// card the user deleted (or that the storage-TTL reconciler removed)
	// cannot be silently re-charged through retry. The ownership clause is
	// defensive: order.CardID is enforced at SetCard time today but binding
	// the retry path to that invariant explicitly costs nothing.
	var card models.Card
	if err := db.Where("id = ? AND user_id = ?", order.CardID, userID).First(&card).Error; err != nil {
		utils.RespondError(w, http.StatusConflict, "CARD_NO_LONGER_AVAILABLE", "saved card is no longer available, add it again to retry", nil)
		return
	}
	if card.IsStorageExpired(time.Now()) {
		if err := expireStoredCard(db, &card); err != nil {
			utils.RespondError(w, http.StatusInternalServerError, "INTERNAL_ERROR", "failed to expire card", nil)
			return
		}
		respondCardStorageExpired(w)
		return
	}

	stripeStart := time.Now()
	confirmed, cerr := paymentintent.Confirm(order.StripePaymentID, &stripe.PaymentIntentConfirmParams{
		PaymentMethod: stripe.String(card.StripePaymentID),
		OffSession:    stripe.Bool(false),
		ReturnURL:     stripe.String(h.appURL + "/post-3ds"),
	})
	stripeDur := time.Since(stripeStart).Seconds()
	if cerr != nil {
		middleware.ObserveStripeRequest("payment_intent_confirm", "error", stripeDur)
		if stripeErr, ok := cerr.(*stripe.Error); ok && stripeErr.Type == stripe.ErrorTypeCard {
			db.Model(&order).Update("status", models.OrderStatusFailed)
			audit.OrderFailed(userID, order.ID, order.Total, stripeErr.Msg)
			middleware.RecordCartConversion("retry_card_declined")
			utils.RespondError(w, http.StatusPaymentRequired, mapStripeCardErrCode(stripeErr.Code), stripeErr.Msg, nil)
			return
		}
		if stripeErr, ok := cerr.(*stripe.Error); ok && stripeErr.Code == "payment_intent_unexpected_state" {
			actual, gerr := paymentintent.Get(order.StripePaymentID, nil)
			if gerr != nil {
				middleware.RecordCartConversion("retry_stripe_error")
				utils.RespondError(w, http.StatusBadGateway, "PAYMENT_FAILED", "payment failed", nil)
				return
			}
			switch actual.Status {
			case stripe.PaymentIntentStatusSucceeded:
				middleware.RecordCartConversion("retry_race_already_done")
				utils.RespondSuccess(w, http.StatusAccepted, map[string]any{
					"order_id": order.ID,
					"status":   "succeeded",
				})
				return
			case stripe.PaymentIntentStatusCanceled:
				db.Model(&order).Update("status", models.OrderStatusFailed)
				audit.OrderFailed(userID, order.ID, order.Total, "payment intent canceled")
				middleware.RecordCartConversion("retry_pi_canceled")
				utils.RespondError(w, http.StatusConflict, "INVALID_STATE", "payment intent is canceled, please start a new order", nil)
				return
			default:
				middleware.RecordCartConversion("retry_pi_unexpected_state")
				utils.RespondError(w, http.StatusBadGateway, "PAYMENT_FAILED", "payment intent in unexpected state", nil)
				return
			}
		}

		middleware.RecordCartConversion("retry_stripe_error")
		utils.RespondError(w, http.StatusBadGateway, "PAYMENT_FAILED", "payment failed", nil)
		return
	}
	middleware.ObserveStripeRequest("payment_intent_confirm", "success", stripeDur)

	// frictionless 3DS on retry, stripe approved without a new challenge.
	// the payment_intent.succeeded webhook owns the order.status=paid
	// transition (matches the initial-flow contract); tell the TUI to keep
	// polling /orders/{id}/status until it lands
	if confirmed.Status == stripe.PaymentIntentStatusSucceeded {
		middleware.RecordCartConversion("retry_frictionless")
		utils.RespondSuccess(w, http.StatusAccepted, map[string]any{
			"order_id": order.ID,
			"status":   "succeeded",
		})
		return
	}

	h.respondRequiresAction(w, &order, card.StripePaymentID, confirmed)
}