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/ssh_key.py

@@ -6,7 +6,6 @@ Security rules:
 - The ``encrypted_private_key`` field uses ``repr=False``.
 """
 
-import base64
 import uuid
 from datetime import UTC, datetime
 
@@ -16,11 +15,9 @@ from app.git.types import SshKeyStatus
 
 
 def encrypt_private_key(raw: bytes) -> str:
-    """Placeholder encryption helper.
+    from app.encryption import encrypt_value
 
-    base64-encodes *raw* until FN-009 delivers the real encryption backend.
-    """
-    return base64.b64encode(raw).decode("ascii")
+    return encrypt_value(raw.decode("utf-8"))
 
 
 class SshKeyPair(BaseModel):