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/models/user_session.py

@@ -76,7 +76,11 @@ class UserSession(Base, UUIDMixin, TimestampMixin):
         """Check if session has expired."""
         from datetime import datetime
 
-        return self.expires_at < datetime.now(UTC)
+        now = datetime.now(UTC)
+        expires_at = self.expires_at
+        if expires_at.tzinfo is None:
+            expires_at = expires_at.replace(tzinfo=UTC)
+        return bool(expires_at < now)
 
     def to_dict(self):
         """Convert session to dictionary for serialization."""