Test that only the image routes send CORS headers (closes #98)

A new server test sends requests with an Origin header through the
server's routes and expects Access-Control-Allow-Origin, set to the
configured origin, on both image routes, a preflight OPTIONS request
included, and no such header on the login and URL generator pages.
It fails for now: the CORS middleware still wraps every route.

The encrypted image path the maintenance tests use is now a named
constant, shared with the new test; what they check is unchanged.

Model: opus-5-5

internal/server/maintenance_internal_test.go

@@ -13,6 +13,10 @@ import (
 // unsignedImagePath is an image URL that carries no signature.
 const unsignedImagePath = "/v1/image/cdn.example.com/cat.jpg/100x100.jpeg"
 
+// encryptedImagePath is an encrypted image URL whose token cannot be
+// decrypted.
+const encryptedImagePath = "/v1/e/token/cat.jpg"
+
 // TestMaintenanceModeRefusesImageRequests verifies that while maintenance
 // mode is on, both image routes answer 503 Service Unavailable with a
 // Retry-After header and the JSON error body the image handlers send.
@@ -28,7 +32,7 @@ func TestMaintenanceModeRefusesImageRequests(t *testing.T) {
 	}{
 		{http.MethodGet, unsignedImagePath},
 		{http.MethodHead, unsignedImagePath},
-		{http.MethodGet, "/v1/e/token/cat.jpg"},
+		{http.MethodGet, encryptedImagePath},
 	}
 
 	for _, tc := range requests {
@@ -95,7 +99,7 @@ func TestImageRequestsServedWithoutMaintenanceMode(t *testing.T) {
 	}{
 		{http.MethodGet, unsignedImagePath, http.StatusUnauthorized},
 		{http.MethodHead, unsignedImagePath, http.StatusUnauthorized},
-		{http.MethodGet, "/v1/e/token/cat.jpg", http.StatusBadRequest},
+		{http.MethodGet, encryptedImagePath, http.StatusBadRequest},
 	}
 
 	for _, tc := range requests {