- Add @pytest_asyncio.fixture db_session: in-memory SQLite via aiosqlite,
expire_on_commit=False, skips gracefully (ImportError) before Plan 03
- Add @pytest_asyncio.fixture async_client: httpx.AsyncClient with
ASGITransport, overrides deps.db.get_db, skips before Plan 03
- Retain all legacy sync fixtures (isolated_data_dir, client, sample_txt,
sample_pdf) unchanged for backward compatibility through Plan 04