test: a handler that flushes a write and never commits it fails its test

Helpers flush and handlers commit — but every test request shared the test's session, so a
follow-up GET or an assertion read the flushed write and the suite stayed green over a handler
whose commit was missing (production's get_db closes the session: the write is rolled back behind
a 200). The db fixture now hands out `db.info["request"]`: the same session, recording each request
that ends normally with a flushed, uncommitted write, and failing the test on it at teardown. All
28 get_db overrides use it (a convention test keeps it so); the full suite passes under it, and
dropping the commit from add_item turns a playlist test red.

backend/tests/test_plex_clear_resume_push.py

@@ -52,7 +52,7 @@ def _clear(db, user, it, monkeypatch, override_dep) -> list[tuple]:
         "push_state_to_plex",
         lambda user_id, item_id, rating_key, action, *rest: pushes.append((rating_key, action)),
     )
-    override_dep(get_db, lambda: db)
+    override_dep(get_db, db.info["request"])
     override_dep(current_user, lambda: user)
     resp = TestClient(app).post(f"/api/plex/item/{it.rating_key}/state", json={"status": "new"})
     assert resp.status_code == 200, resp.text