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_channel_backfill.py

@@ -40,7 +40,7 @@ def test_channel_detail_returns_normal_shorts_live_counts(db, monkeypatch, overr
 
     # The request gate 503s an unconfigured instance; flip the process-level flag (auto-restored).
     monkeypatch.setattr(state, "_configured_cache", True)
-    override_dep(get_db, lambda: db)
+    override_dep(get_db, db.info["request"])
     override_dep(current_user, lambda: me)
     resp = TestClient(app).get(f"/api/channels/{ch.id}")
 
@@ -65,7 +65,7 @@ def test_channel_detail_surfaces_subscription_hidden_flag(db, monkeypatch, hidde
     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: me)
     resp = TestClient(app).get(f"/api/channels/{ch.id}")
 
@@ -81,7 +81,7 @@ def test_channel_detail_hidden_defaults_false_when_not_subscribed(db, monkeypatc
     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: me)
     resp = TestClient(app).get(f"/api/channels/{ch.id}")
 
@@ -111,7 +111,7 @@ def _patch_deep_requested(db, monkeypatch, override_dep, user):
     # background.add_task runs after the response in TestClient; record what would be scheduled.
     monkeypatch.setattr(channels_route, "backfill_channel_now", lambda *a, **k: calls.append(a))
     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).patch("/api/channels/UC_wire", json={"deep_requested": True})
     return resp, calls