server: limit wildcard CORS to the public routes (closes #100)

The CORS wildcard was global, so it also covered the Basic-Auth
protected /metrics, which REPO_POLICIES.md forbids, and it allowed
POST, PUT and DELETE, which no route serves, plus the Authorization
and X-CSRF-Token headers. CORS now sits on a router holding only the
public routes and allows GET and OPTIONS with the Accept and
Content-Type headers. /metrics gets no CORS at all.

Both are mounted routers rather than a Group: chi answers OPTIONS on
a Group's route with 405 before its middleware runs, and any method
/metrics does not register would otherwise fall through to the public
router. So every method on /metrics now meets Basic Auth first, and
/metrics/ is served like /metrics.

Model: opus-5-5

internal/server/routes.go

@@ -23,14 +23,20 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(chimw.RequestID)
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
-	s.router.Use(s.mw.CORS())
 	s.router.Use(chimw.Timeout(requestTimeout))
 
+	// Public, unauthenticated, read-only routes, the only ones
+	// REPO_POLICIES.md allows wildcard CORS on. CORS is middleware of
+	// this whole router, not of a Group, so that it also answers
+	// OPTIONS preflight requests, which no route here registers.
+	public := chi.NewRouter()
+	public.Use(s.mw.CORS())
+
 	// Dashboard (read-only web UI)
-	s.router.Get("/", s.handlers.HandleDashboard())
+	public.Get("/", s.handlers.HandleDashboard())
 
 	// Static assets (embedded CSS/JS)
-	s.router.Mount(
+	public.Mount(
 		"/s",
 		http.StripPrefix(
 			"/s",
@@ -39,27 +45,30 @@ func (s *Server) SetupRoutes() {
 	)
 
 	// Health check (standard well-known path)
-	s.router.Get(
+	public.Get(
 		"/.well-known/healthcheck",
 		s.handlers.HandleHealthCheck(),
 	)
 
 	// Legacy health check (keep for backward compatibility)
-	s.router.Get("/health", s.handlers.HandleHealthCheck())
+	public.Get("/health", s.handlers.HandleHealthCheck())
 
 	// API v1 routes
-	s.router.Route("/api/v1", func(r chi.Router) {
+	public.Route("/api/v1", func(r chi.Router) {
 		r.Get("/status", s.handlers.HandleStatus())
 	})
 
-	// Metrics endpoint (optional, with basic auth)
+	s.router.Mount("/", public)
+
+	// Metrics endpoint (optional, with basic auth) and no CORS: a
+	// Prometheus scraper is not a browser. It is mounted rather than
+	// added with Get so that every method on /metrics, OPTIONS
+	// included, ends here instead of falling through to the public
+	// router and its CORS.
 	if s.params.Config.MetricsUsername != "" {
-		s.router.Group(func(r chi.Router) {
-			r.Use(s.mw.MetricsAuth())
-			r.Get(
-				"/metrics",
-				promhttp.Handler().ServeHTTP,
-			)
-		})
+		metrics := chi.NewRouter()
+		metrics.Use(s.mw.MetricsAuth())
+		metrics.Get("/", promhttp.Handler().ServeHTTP)
+		s.router.Mount("/metrics", metrics)
 	}
 }