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_override_dep_convention.py

@@ -21,3 +21,17 @@ def test_no_test_writes_dependency_overrides_directly():
         if "dependency_overrides" in line
     ]
     assert offenders == [], "use the override_dep fixture: " + ", ".join(offenders)
+
+
+def test_a_request_gets_the_session_that_checks_its_commit():
+    """`get_db` is overridden with `db.info["request"]` — the test session plus the check that the
+    handler committed what it flushed (conftest `_request_dep`). A bare `lambda: db` hands the route
+    the same session without the check, and a handler that forgot its commit passes again."""
+    offenders = [
+        f"{p.name}:{n}"
+        for p in sorted(TESTS.rglob("*.py"))
+        if p.name != Path(__file__).name
+        for n, line in enumerate(p.read_text(encoding="utf-8").splitlines(), 1)
+        if "override_dep(get_db," in line and 'override_dep(get_db, db.info["request"])' not in line
+    ]
+    assert offenders == [], 'override get_db with db.info["request"]: ' + ", ".join(offenders)