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_facets.py

@@ -33,7 +33,7 @@ def test_facets_returns_studio_vocabulary(db, monkeypatch, override_dep):
     db.add(user)
     db.commit()
     monkeypatch.setattr(state, "_configured_cache", True)
-    override_dep(get_db, lambda: db)
+    override_dep(get_db, db.info["request"])
     override_dep(current_user, lambda: user)
     resp = TestClient(app).get("/api/plex/facets?scope=movie")
     assert resp.status_code == 200, resp.text
@@ -58,7 +58,7 @@ def test_facets_survives_json_null_genres(db, monkeypatch, override_dep):
     db.execute(text("UPDATE plex_shows SET genres = 'null'::jsonb WHERE rating_key = 's1'"))
     db.commit()
 
-    override_dep(get_db, lambda: db)
+    override_dep(get_db, db.info["request"])
     override_dep(current_user, lambda: user)
     client = TestClient(app)  # no context-manager: skip lifespan (scheduler/HLS sweep)
     resp = client.get("/api/plex/facets?scope=show")