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/utils/test_utils.py

@@ -65,10 +65,10 @@ async def setup_async_test_db():
     async with test_engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
 
-    AsyncTestingSessionLocal = sessionmaker(
+    AsyncTestingSessionLocal = sessionmaker(  # pyright: ignore[reportCallIssue]
         autocommit=False,
         autoflush=False,
-        bind=test_engine,
+        bind=test_engine,  # pyright: ignore[reportArgumentType]
         expire_on_commit=False,
         class_=AsyncSession,
     )