docs(docs): update docs to get rid of gopls warnings. update some code to newer standards (#49)

api/doc.go

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+// Command api is the HTTP backend: it boots the router, bolts on all the
+// middleware, and answers the requests the TUI throws at it.
+package main

api/handlers/doc.go

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+// Package handlers is where the real work happens, one function per endpoint:
+// carts, orders, products, addresses, cards, webhooks, and the rest.
+package handlers

api/middleware/doc.go

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+// Package middleware holds the stuff that runs on every request before a
+// handler ever sees it: auth, CORS, logging, rate limiting, metrics, and a
+// recovery net for panics.
+package middleware

api/routes/doc.go

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+// Package routes is the switchboard: it maps every URL to the handler that
+// knows what to do with it.
+package routes