Initial implementation of OAuth models, endpoints, and migrations

- Added models for `OAuthClient`, `OAuthState`, and `OAuthAccount`.
- Created Pydantic schemas to support OAuth flows, client management, and linked accounts.
- Implemented skeleton endpoints for OAuth Provider mode: authorization, token, and revocation.
- Updated router imports to include new `/oauth` and `/oauth/provider` routes.
- Added Alembic migration script to create OAuth-related database tables.
- Enhanced `users` table to allow OAuth-only accounts by making `password_hash` nullable.

backend/app/services/__init__.py

@@ -0,0 +1,5 @@
+# app/services/__init__.py
+from .auth_service import AuthService
+from .oauth_service import OAuthService
+
+__all__ = ["AuthService", "OAuthService"]