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/crud/__init__.py

@@ -1,6 +1,14 @@
 # app/crud/__init__.py
+from .oauth import oauth_account, oauth_client, oauth_state
 from .organization import organization
 from .session import session as session_crud
 from .user import user
 
-__all__ = ["organization", "session_crud", "user"]
+__all__ = [
+    "oauth_account",
+    "oauth_client",
+    "oauth_state",
+    "organization",
+    "session_crud",
+    "user",
+]