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/api/main.py

@@ -1,9 +1,21 @@
 from fastapi import APIRouter
 
-from app.api.routes import admin, auth, organizations, sessions, users
+from app.api.routes import (
+    admin,
+    auth,
+    oauth,
+    oauth_provider,
+    organizations,
+    sessions,
+    users,
+)
 
 api_router = APIRouter()
 api_router.include_router(auth.router, prefix="/auth", tags=["Authentication"])
+api_router.include_router(oauth.router, prefix="/oauth", tags=["OAuth"])
+api_router.include_router(
+    oauth_provider.router, prefix="/oauth", tags=["OAuth Provider"]
+)
 api_router.include_router(users.router, prefix="/users", tags=["Users"])
 api_router.include_router(sessions.router, prefix="/sessions", tags=["Sessions"])
 api_router.include_router(admin.router, prefix="/admin", tags=["Admin"])