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/models/__init__.py

@@ -7,6 +7,11 @@ Imports all models to ensure they're registered with SQLAlchemy.
 from app.core.database import Base
 
 from .base import TimestampMixin, UUIDMixin
+
+# OAuth models
+from .oauth_account import OAuthAccount
+from .oauth_client import OAuthClient
+from .oauth_state import OAuthState
 from .organization import Organization
 
 # Import models
@@ -16,6 +21,9 @@ from .user_session import UserSession
 
 __all__ = [
     "Base",
+    "OAuthAccount",
+    "OAuthClient",
+    "OAuthState",
     "Organization",
     "OrganizationRole",
     "TimestampMixin",