refactor(backend): migrate type checking from mypy to pyright

Replace mypy>=1.8.0 with pyright>=1.1.390. Remove all [tool.mypy] and
[tool.pydantic-mypy] sections from pyproject.toml and add
pyrightconfig.json (standard mode, SQLAlchemy false-positive rules
suppressed globally).

Fixes surfaced by pyright:
- Remove unreachable except AuthError clauses in login/login_oauth (same class as AuthenticationError)
- Fix Pydantic v2 list Field: min_items/max_items → min_length/max_length
- Split OAuthProviderConfig TypedDict into required + optional(email_url) inheritance
- Move JWTError/ExpiredSignatureError from lazy try-block imports to module level
- Add timezone-aware guard to UserSession.is_expired to match sibling models
- Fix is_active: bool → bool | None in three organization repo signatures
- Initialize search_filter = None before conditional block (possibly unbound fix)
- Add bool() casts to model is_expired and repo is_active/is_superuser returns
- Restructure except (JWTError, Exception) into separate except clauses

backend/app/repositories/user.py

@@ -256,11 +256,11 @@ class UserRepository(BaseRepository[User, UserCreate, UserUpdate]):
 
     def is_active(self, user: User) -> bool:
         """Check if user is active."""
-        return user.is_active
+        return bool(user.is_active)
 
     def is_superuser(self, user: User) -> bool:
         """Check if user is a superuser."""
-        return user.is_superuser
+        return bool(user.is_superuser)
 
 
 # Singleton instance