fix(fc3b): ruff I001 (blank-line) + F841 (unused crypto binding)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/__init__.py

@@ -10,7 +10,6 @@ from .config import get_config
 from .frontend import frontend_bp
 from .services.credential_crypto import CredentialCrypto
 
-
 _CREDENTIAL_KEY_PATH = Path("/images/secrets/credential_key.b64")
 
 

tests/test_credential_crypto.py

@@ -9,13 +9,12 @@ from backend.app.services.credential_crypto import (
     InvalidCredentialBlob,
 )
 
-
 pytestmark = pytest.mark.integration
 
 
 def test_load_or_create_writes_key_with_mode_0600(tmp_path):
     key_path = tmp_path / "secrets" / "credential_key.b64"
-    crypto = CredentialCrypto(key_path)
+    CredentialCrypto(key_path)
     assert key_path.exists()
     mode = stat.S_IMODE(os.stat(key_path).st_mode)
     assert mode == 0o600