Clean up Alembic migrations

- Removed outdated and redundant Alembic migration files to streamline the migration directory. This improves maintainability and eliminates duplicate or unused scripts.

backend/app/models/user_session.py

@@ -22,6 +22,9 @@ class UserSession(Base, UUIDMixin, TimestampMixin):
 
     Each time a user logs in from a device, a new session is created.
     Sessions are identified by the refresh token JTI (JWT ID).
+
+    Performance indexes (defined in migration 0002_add_performance_indexes.py):
+    - ix_perf_user_sessions_expires: expires_at WHERE is_active = true
     """
 
     __tablename__ = "user_sessions"