fix: resolve CI failures — gofmt, unused functions, missing test helpers

- Run gofmt on user schema, config test, group handler
- Remove unused mergeGroupIDs function
- Restore shared test helpers (newJSONResponse, queuedHTTPUpstream)
  that were in deleted Sora test file

backend/internal/config/config_test.go

@@ -1554,7 +1554,6 @@ func TestValidateConfig_LogRequiredAndRotationBounds(t *testing.T) {
 	}
 }
 
-
 func TestLoad_DefaultGatewayUsageRecordConfig(t *testing.T) {
 	resetViperWithJWTSecret(t)
 	cfg, err := Load()