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/models/credential.py

@@ -0,0 +1,16 @@
+from typing import TYPE_CHECKING
+
+from sqlalchemy import String, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base, TimestampMixin, UUIDMixin
+
+if TYPE_CHECKING:
+    pass
+
+
+class Credential(Base, UUIDMixin, TimestampMixin):
+    __tablename__ = "credential"
+
+    kind: Mapped[str] = mapped_column(String(50))
+    encrypted_payload: Mapped[str] = mapped_column(Text)