Update session cleanup logic, pyproject.toml, and Makefile for consistency and improved tooling support

- Replaced `not UserSession.is_active` with `UserSession.is_active == False` in cleanup queries for explicit comparison.
- Added `mypy` overrides for `app.alembic` and external libraries (`starlette`).
- Refactored `Makefile` to use virtual environment binaries for commands like `ruff`, `mypy`, and `pytest`.

backend/app/crud/session.py

@@ -309,7 +309,7 @@ class CRUDSession(CRUDBase[UserSession, SessionCreate, SessionUpdate]):
             # Use bulk DELETE with WHERE clause - single query
             stmt = delete(UserSession).where(
                 and_(
-                    not UserSession.is_active,
+                    UserSession.is_active == False,  # noqa: E712
                     UserSession.expires_at < now,
                     UserSession.created_at < cutoff_date,
                 )
@@ -356,7 +356,7 @@ class CRUDSession(CRUDBase[UserSession, SessionCreate, SessionUpdate]):
             stmt = delete(UserSession).where(
                 and_(
                     UserSession.user_id == uuid_obj,
-                    not UserSession.is_active,
+                    UserSession.is_active == False,  # noqa: E712
                     UserSession.expires_at < now,
                 )
             )