fix(worker): keep DLQ failures within the canonical taxonomy

tests/test_worker.py

@@ -262,10 +262,10 @@ class TestProcessingWorker(unittest.TestCase):
         msg = MockKafkaMessage(json.dumps(payload).encode("utf-8"))
         worker.process_single_message(msg)
 
-        # Record should have fallen back to DLQ with ch_insert_failed
+        # Record should have fallen back to DLQ with normalization_error
         self.assertEqual(len(self.dlq_producer.messages), 1)
         dlq_data = json.loads(self.dlq_producer.messages[0]["value"].decode("utf-8"))
-        self.assertEqual(dlq_data["failure_reason"], "ch_insert_failed")
+        self.assertEqual(dlq_data["failure_reason"], "normalization_error")
 
     def test_internal_stats_and_metrics_endpoints(self):
         # Verify /internal/stats