fix(services): PEP 695 type params for get_or_create (ruff UP047)

CI lint flagged UP047 — use the native generic syntax def get_or_create[T](...)
instead of typing.TypeVar on Python 3.14.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/db_helpers.py

@@ -15,16 +15,13 @@ sync/async boundary; the importer one stays as the lone sync consumer.
 from __future__ import annotations
 
 from collections.abc import Awaitable, Callable
-from typing import TypeVar
 
 from sqlalchemy import Select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
-T = TypeVar("T")
 
-
-async def get_or_create(
+async def get_or_create[T](
     session: AsyncSession,
     select_stmt: Select,
     factory: Callable[[], Awaitable[T]],