test(tasks): cover cleanup_message_events task body

Adds skipped-when-no-POSTGRES_URI and happy-path coverage for the
Celery janitor. The skipped path returns the documented short-circuit
shape without touching the repo. The happy path seeds a backdated
row, runs the task against the pg_conn fixture, and asserts the
retention window's row is deleted while in-window rows survive.
Mirrors the TestCleanupPendingToolState pattern.

tests/api/user/test_tasks.py

@@ -428,6 +428,93 @@ class TestCleanupPendingToolState:
         }
 
 
+class TestCleanupMessageEventsTask:
+    """Retention janitor delegates to MessageEventsRepository.cleanup_older_than."""
+
+    @pytest.mark.unit
+    def test_skips_when_postgres_uri_missing(self, monkeypatch):
+        from application.api.user.tasks import cleanup_message_events
+        from application.core.settings import settings
+
+        monkeypatch.setattr(settings, "POSTGRES_URI", None, raising=False)
+
+        result = cleanup_message_events.run()
+        assert result == {"deleted": 0, "skipped": "POSTGRES_URI not set"}
+
+    @pytest.mark.unit
+    def test_deletes_rows_past_retention_window(self, pg_conn, monkeypatch):
+        import uuid
+
+        from sqlalchemy import text as _text
+
+        from application.api.user.tasks import cleanup_message_events
+        from application.core.settings import settings
+        from application.storage.db.repositories.message_events import (
+            MessageEventsRepository,
+        )
+
+        # Seed parent rows so the FK on message_events holds.
+        user_id = f"user-{uuid.uuid4().hex[:8]}"
+        conv_id = uuid.uuid4()
+        msg_id = uuid.uuid4()
+        pg_conn.execute(
+            _text("INSERT INTO users (user_id) VALUES (:u)"),
+            {"u": user_id},
+        )
+        pg_conn.execute(
+            _text(
+                "INSERT INTO conversations (id, user_id, name) "
+                "VALUES (:id, :u, 'test')"
+            ),
+            {"id": conv_id, "u": user_id},
+        )
+        pg_conn.execute(
+            _text(
+                "INSERT INTO conversation_messages (id, conversation_id, "
+                "user_id, position) VALUES (:id, :c, :u, 0)"
+            ),
+            {"id": msg_id, "c": conv_id, "u": user_id},
+        )
+
+        repo = MessageEventsRepository(pg_conn)
+        repo.record(str(msg_id), 0, "answer", {"chunk": "stale"})
+        repo.record(str(msg_id), 1, "answer", {"chunk": "fresh"})
+        # Backdate seq=0 past the default 14-day retention so the
+        # janitor catches it; seq=1 stays at "now" and must survive.
+        pg_conn.execute(
+            _text(
+                "UPDATE message_events SET created_at = now() - interval '20 days' "
+                "WHERE message_id = CAST(:id AS uuid) AND sequence_no = 0"
+            ),
+            {"id": str(msg_id)},
+        )
+
+        monkeypatch.setattr(
+            settings, "POSTGRES_URI", "postgresql://stub", raising=False
+        )
+
+        @contextmanager
+        def _fake_begin():
+            yield pg_conn
+
+        fake_engine = MagicMock()
+        fake_engine.begin = _fake_begin
+
+        with patch(
+            "application.storage.db.engine.get_engine",
+            return_value=fake_engine,
+        ):
+            result = cleanup_message_events.run()
+
+        assert result == {
+            "deleted": 1,
+            "ttl_days": settings.MESSAGE_EVENTS_RETENTION_DAYS,
+        }
+        # Only the fresh row survives.
+        rows = repo.read_after(str(msg_id))
+        assert [r["sequence_no"] for r in rows] == [1]
+
+
 class TestIngestIdempotency:
     """Same short-circuit applies to the ingest task path."""