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/middleware/middleware.go

@@ -223,17 +223,14 @@ func realIP(r *http.Request) string {
 	return addr
 }
 
-// CORS returns CORS middleware.
+// CORS returns middleware that lets any origin read a response. It is
+// for the public, read-only routes only, so it allows only the
+// methods those routes serve and no Authorization header.
 func (m *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{
-		AllowedOrigins: []string{"*"},
-		AllowedMethods: []string{
-			"GET", "POST", "PUT", "DELETE", "OPTIONS",
-		},
-		AllowedHeaders: []string{
-			"Accept", "Authorization",
-			"Content-Type", "X-CSRF-Token",
-		},
+		AllowedOrigins:   []string{"*"},
+		AllowedMethods:   []string{"GET", "OPTIONS"},
+		AllowedHeaders:   []string{"Accept", "Content-Type"},
 		ExposedHeaders:   []string{"Link"},
 		AllowCredentials: false,
 		MaxAge:           corsMaxAge,