fix: ruff warnings

src/fastapi_toolsets/db/m2m.py

@@ -1,6 +1,6 @@
 """Many-to-Many association-table helpers (direct, without loading collections)."""
 
-from typing import Any, TypeVar, cast
+from typing import Any, cast
 
 from sqlalchemy import ColumnElement, Table, delete, tuple_
 from sqlalchemy.dialects.postgresql import insert as pg_insert
@@ -8,8 +8,6 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import DeclarativeBase, QueryableAttribute
 from sqlalchemy.orm.relationships import RelationshipProperty
 
-_M = TypeVar("_M", bound=DeclarativeBase)
-
 
 def _m2m_prop(rel_attr: QueryableAttribute) -> tuple[RelationshipProperty, Table]:  # type: ignore[type-arg]
     """Return the validated M2M RelationshipProperty and its secondary table.