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/pyproject.toml

@@ -54,6 +54,9 @@ dependencies = [
     "passlib==1.7.4",
     "bcrypt==4.2.1",
     "cryptography==44.0.1",
+
+    # OAuth authentication
+    "authlib>=1.3.0",
 ]
 
 # Development dependencies
@@ -243,6 +246,10 @@ ignore_missing_imports = true
 module = "starlette.*"
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = "authlib.*"
+ignore_missing_imports = true
+
 # SQLAlchemy ORM models - Column descriptors cause type confusion
 [[tool.mypy.overrides]]
 module = "app.models.*"