refactor: replace routeNormalizer with context pointer for route labeling

[Brunotlps]

Summary

• routeNormalizer in the logging middleware independently reimplemented the same sort+match logic as the router
• Any future change to routing behavior (trailing slash, exact-match, regex) would silently diverge Prometheus labels from the actual routing decision — no error, no warning
• The middleware now allocates a *string and stores it in the request context under ctxkey.RouteLabelKey before calling next.ServeHTTP
• The router writes the matched prefix through that pointer after route selection
• A pointer (not a value) is required because r.WithContext returns a new *http.Request local to the callee — a plain string would not propagate back to the middleware
• New package internal/ctxkey defines the shared key type, avoiding import cycles between middleware and router

Files changed

• internal/ctxkey/ctxkey.go — new shared context key package
• internal/middleware/logging.go — removed routeNormalizer, reads label via context pointer
• internal/middleware/logging_test.go — replaced routeNormalizer unit tests with behavior tests for the context flow
• internal/router/router.go — writes matched prefix to context pointer after route selection
• cmd/gateway/main.go — removed routePrefixes slice and extra argument from Logging()

Test plan

• go build ./... succeeds
• go test -race ./... passes — 4 new tests covering context pointer flow, fallback to "unknown", plain context compatibility

Closes #10

🤖 Generated with Claude Code