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_subtitle_auth.py

@@ -28,7 +28,7 @@ def test_a_suspended_account_cannot_drive_the_subtitle_extractor(
     db, make_user, override_dep, monkeypatch, tmp_path
 ):
     user = make_user(is_suspended=True)
-    override_dep(get_db, lambda: db)
+    override_dep(get_db, db.info["request"])
     monkeypatch.setattr(app_state, "_configured_cache", True)
     # A validly SIGNED cookie for that account — which is all the old gate looked at.
     for mod in (auth, playback):