feat: implement all 4 ORM models (SshKey, Server, Repo, SyncLog)

- Created SshKey model with encrypted private key storage
- Created Server model with Gitea configuration and SshKey relationship
- Created Repo model with repository mirror info and Server relationship
- Created SyncLog model with sync operation logs and Repo relationship
- Updated models/__init__.py to export all models
- All models use Integer (Unix timestamp) for datetime fields
- Proper bidirectional relationships using back_populates
- Added comprehensive test suite for all models and relationships

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -1,3 +1,7 @@
 from app.database import Base
+from app.models.ssh_key import SshKey
+from app.models.server import Server
+from app.models.repo import Repo
+from app.models.sync_log import SyncLog
 
-__all__ = ['Base']
+__all__ = ['Base', 'SshKey', 'Server', 'Repo', 'SyncLog']