refactor(audit): drop dead persist call + harden cart_rejected test capture

api/handlers/cart.go

@@ -382,6 +382,7 @@ func (h *CartHandler) ConvertCart(w http.ResponseWriter, r *http.Request) {
 
 	if h.maxOrderCents > 0 && total > h.maxOrderCents {
 		middleware.RecordCartConversion("validation_over_limit")
+		audit.CartRejected(userID, total, h.maxOrderCents)
 		utils.RespondError(w, http.StatusBadRequest, "CART_OVER_LIMIT",
 			fmt.Sprintf("order total must be at most $%.2f", float64(h.maxOrderCents)/100),
 			map[string]any{

api/handlers/cart_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -746,6 +747,19 @@ func TestConvertCartLimit(t *testing.T) {
 				t.Fatalf("SetCard: %d %s", w.Code, w.Body.String())
 			}
 
+			// Capture audit slog ONLY for ConvertCart's emit window. Placed
+			// here (not at top of t.Run) so setup INFO logs from
+			// database.Connect/Migrate/Seed don't pollute the buffer with
+			// non-audit records - json.Unmarshal below requires a single
+			// JSON object, not multi-document input. slog.Default is
+			// process-global - do NOT add t.Parallel() to subtests here.
+			var auditBuf bytes.Buffer
+			if tc.expectOverCap {
+				prev := slog.Default()
+				slog.SetDefault(slog.New(slog.NewJSONHandler(&auditBuf, nil)))
+				t.Cleanup(func() { slog.SetDefault(prev) })
+			}
+
 			// Convert
 			req = authRequest("POST", "/api/v1/cart/convert", nil, user.ID)
 			w = httptest.NewRecorder()
@@ -772,6 +786,27 @@ func TestConvertCartLimit(t *testing.T) {
 				if got, _ := resp.Error.Details["total_cents"].(float64); int(got) != tc.quantity*pinnedPrice {
 					t.Errorf("total_cents in details: want %d, got %v", tc.quantity*pinnedPrice, resp.Error.Details["total_cents"])
 				}
+
+				if auditBuf.Len() == 0 {
+					t.Fatalf("audit: expected cart_rejected slog line, got nothing")
+				}
+
+				var rec map[string]any
+				if err := json.Unmarshal(auditBuf.Bytes(), &rec); err != nil {
+					t.Fatalf("audit: decode slog record: %v\nraw: %s", err, auditBuf.String())
+				}
+				if got := rec["event"]; got != "cart_rejected" {
+					t.Errorf("audit event: want cart_rejected, got %v", got)
+				}
+				if n, _ := rec["user_id"].(float64); uint(n) != user.ID {
+					t.Errorf("audit user_id: want %d, got %v", user.ID, rec["user_id"])
+				}
+				if n, _ := rec["total_cents"].(float64); int(n) != tc.quantity*pinnedPrice {
+					t.Errorf("audit total_cents: want %d, got %v", tc.quantity*pinnedPrice, rec["total_cents"])
+				}
+				if n, _ := rec["cap_cents"].(float64); int(n) != tc.capCents {
+					t.Errorf("audit cap_cents: want %d, got %v", tc.capCents, rec["cap_cents"])
+				}
 			}
 		})
 	}

pkg/audit/audit.go

@@ -24,6 +24,12 @@ const EventOrderRequiresAction = "order_requires_action"
 // stays decoupled from sweep activity
 const EventOrder3DSAbandoned = "order_3ds_abandoned"
 
+// EventCartRejected is the audit event-type string emitted when the per-order
+// spend cap blocks a checkout BEFORE an order row is created. The event lives
+// in slog only, persist() no-ops on zero order id, which is the correct shape
+// here (cap rejection is upstream of order state). See caveat #13.
+const EventCartRejected = "cart_rejected"
+
 type eventType string
 
 const (
@@ -36,6 +42,7 @@ const (
 	eventOrderFailed            eventType = "order_failed"
 	eventOrderRequiresAction    eventType = EventOrderRequiresAction
 	eventOrder3DSAbandoned      eventType = EventOrder3DSAbandoned
+	eventCartRejected           eventType = EventCartRejected
 	eventOrderRefunded          eventType = "order_refunded"
 	eventPaymentCritical        eventType = "payment_critical"
 	eventLabelPurchased         eventType = "label_purchased"
@@ -46,19 +53,21 @@ const (
 )
 
 type event struct {
-	Event     eventType
-	UserID    uint
-	OrderID   uint
-	CardID    uint
-	Amount    int
-	StripeID  string
-	CardLast4 string
-	CardBrand string
-	Carrier   string
-	Tracking  string
-	Status    string
-	Error     string
-	Actor     string
+	Event      eventType
+	UserID     uint
+	OrderID    uint
+	CardID     uint
+	Amount     int
+	StripeID   string
+	CardLast4  string
+	CardBrand  string
+	Carrier    string
+	Tracking   string
+	Status     string
+	Error      string
+	Actor      string
+	TotalCents int
+	CapCents   int
 }
 
 func (e event) attrs() []any {
@@ -99,6 +108,12 @@ func (e event) attrs() []any {
 	if e.Actor != "" {
 		a = append(a, "actor", e.Actor)
 	}
+	if e.TotalCents != 0 {
+		a = append(a, "total_cents", e.TotalCents)
+	}
+	if e.CapCents != 0 {
+		a = append(a, "cap_cents", e.CapCents)
+	}
 	return a
 }
 
@@ -349,3 +364,19 @@ func Order3DSAbandoned(userID, orderID uint, paymentIntentID string) {
 	slog.Info("audit", e.attrs()...)
 	persist(orderID, eventOrder3DSAbandoned, e)
 }
+
+// CartRejected records the per-order cap blocking a checkout BEFORE an
+// order row is created. Slog-only by design: cap rejection runs upstream
+// of order creation so there is no order id to scope a persistent
+// OrderEvent row. persist() is intentionally not called - future refactors
+// of the persist() zero-id guard must not silently start writing rows here.
+// Counterpart to the validation_over_limit Prometheus counter.
+func CartRejected(userID uint, totalCents, capCents int) {
+	e := event{
+		Event:      eventCartRejected,
+		UserID:     userID,
+		TotalCents: totalCents,
+		CapCents:   capCents,
+	}
+	slog.Info("audit", e.attrs()...)
+}

pkg/audit/audit_test.go

@@ -0,0 +1,41 @@
+package audit
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"strings"
+	"testing"
+)
+
+func TestCartRejected(t *testing.T) {
+	var buf bytes.Buffer
+	prev := slog.Default()
+	slog.SetDefault(slog.New(slog.NewJSONHandler(&buf, nil)))
+	t.Cleanup(func() { slog.SetDefault(prev) })
+
+	CartRejected(42, 25000, 20000)
+
+	raw := strings.TrimSpace(buf.String())
+	if raw == "" {
+		t.Fatalf("CartRejected: no slog output captured")
+	}
+
+	var rec map[string]any
+	if err := json.Unmarshal([]byte(raw), &rec); err != nil {
+		t.Fatalf("decode slog record: %v\nraw: %s", err, raw)
+	}
+
+	if got, want := rec["event"], "cart_rejected"; got != want {
+		t.Errorf("event: want %q, got %v", want, got)
+	}
+	if n, _ := rec["user_id"].(float64); int(n) != 42 {
+		t.Errorf("user_id want 42, got %v", rec["user_id"])
+	}
+	if n, _ := rec["total_cents"].(float64); int(n) != 25000 {
+		t.Errorf("total_cents: want 25000, got %v", rec["total_cents"])
+	}
+	if n, _ := rec["cap_cents"].(float64); int(n) != 20000 {
+		t.Errorf("cap_cents: want 20000, got %v", rec["cap_cents"])
+	}
+}