fix(storage): emit origin-relative object URLs

Build signed object URLs as path-only (/api/objects/...) instead of
prefixing APP_BASE_URL, so media loads from whatever host or reverse
proxy served the app. Remove the now-unused base URL from the signer and
drop the APP_BASE_URL / Server.BaseURL config.

backend/internal/photos/helpers_test.go

@@ -106,7 +106,7 @@ func newTestService(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend
 	if endpoint == "" {
 		t.Skip("S3_ENDPOINT not set")
 	}
-	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour, 10*time.Minute)
+	signer, err := storage.NewSigner("test-secret", time.Hour, 10*time.Minute)
 	if err != nil {
 		t.Skipf("signer init: %v", err)
 	}
@@ -128,7 +128,7 @@ func newTestService(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend
 // storage backend so the full flow can run without external infrastructure.
 func newTestServiceFS(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend) {
 	t.Helper()
-	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour, 10*time.Minute)
+	signer, err := storage.NewSigner("test-secret", time.Hour, 10*time.Minute)
 	if err != nil {
 		t.Fatalf("storage init: %v", err)
 	}