feat(FN-007): implement credential storage with Fernet encryption

- Add DatabaseCredentialStorage with async CRUD operations
- Create Credential SQLAlchemy model with encrypted values
- Update GitCredential and AccessTokenCredential to support async
- Fix SSH key encryption to use Fernet instead of base64 placeholder

apps/api/app/git/credentials.py

@@ -40,13 +40,13 @@ class CredentialStorage(abc.ABC):
     """Abstract storage backend for :class:`GitCredential` records."""
 
     @abc.abstractmethod
-    def create(self, credential: GitCredential) -> uuid.UUID:
+    async def create(self, credential: GitCredential) -> uuid.UUID:
         """Persist *credential* and return its ID."""
 
     @abc.abstractmethod
-    def get(self, credential_id: uuid.UUID) -> GitCredential | None:
+    async def get(self, credential_id: uuid.UUID) -> GitCredential | None:
         """Retrieve a credential by ID, or ``None`` if not found."""
 
     @abc.abstractmethod
-    def delete(self, credential_id: uuid.UUID) -> None:
+    async def delete(self, credential_id: uuid.UUID) -> None:
         """Remove a credential by ID."""