Stop allowing credentialed cross-origin requests (closes #261)

CORS() allowed every origin with credentials, so a page on another
origin of the same site could read what the signed-in user sees. It
now uses the options of the CONVENTIONS.md template: every origin as
*, no credentials. It applies only to GET /api/v1/server and
GET /.well-known/healthcheck.json, the public read-only routes, since
REPO_POLICIES.md forbids * on authenticated routes. Every other route
sends no Access-Control-Allow-* header.

Model: opus-5-5

internal/server/routes.go

@@ -30,7 +30,6 @@ func (srv *Server) SetupRoutes() {
 		srv.router.Use(srv.mw.Metrics())
 	}
 
-	srv.router.Use(srv.mw.CORS())
 	srv.router.Use(srv.mw.CSP())
 	srv.router.Use(middleware.Timeout(routeTimeout))
 
@@ -45,7 +44,7 @@ func (srv *Server) SetupRoutes() {
 	}
 
 	// Health check.
-	srv.router.Get(
+	srv.router.With(srv.mw.CORS()).Get(
 		"/.well-known/healthcheck.json",
 		srv.handlers.HandleHealthCheck(),
 	)
@@ -69,7 +68,7 @@ func (srv *Server) SetupRoutes() {
 }
 
 func (srv *Server) setupAPIv1(router chi.Router) {
-	router.Get(
+	router.With(srv.mw.CORS()).Get(
 		"/server",
 		srv.handlers.HandleServerInfo(),
 	)