fix(infra): correlate the delivery on the object URL, not the record reference (refs #152)

The publish chain works — the sink received it:

  {"kanaal": "objecten", "resource": "object", "kenmerken": {"objectType": "…"},
   "hoofdObject": "http://objecten.local:8000/api/v2/objects/a68d4c46-…", …}

The check just looked for the wrong thing. An NRC notification carries hoofdObject /
resourceUrl and kenmerken — never the record data — so the `reference` inside the
RegisterRecord it wrote was never going to appear in the delivered message. Grep the sink
for the object URL instead, which is what identifies the write.

infra/objecten-notifications-check.py

@@ -4,7 +4,7 @@
 Registers an abonnement on the `objecten` kanaal pointing at the webhook sink, then writes a
 RegisterRecord object exactly as the ACL's ObjectenGateway does (S-19a). The caller
 (run-objecten-notifications-check.sh) watches the sink for the delivery — this only sets it up,
-and prints `REFERENCE <value>` for the caller to grep on.
+and prints `OBJECT_URL <url>` for the caller to grep on.
 
 Delivery exercises the whole chain: Objecten → its celery worker → NRC → nrc-beat → the callback.
 Anything missing (broker, worker, kanaal, notifications config) shows up as a non-delivery.
@@ -111,7 +111,9 @@ def main():
         },
     }, crs=True)
     print(f">> wrote RegisterRecord {created['url']}")
-    print(f"REFERENCE {reference}")
+    # An NRC notification carries no record data — only hoofdObject/resourceUrl — so the object
+    # URL, not the reference in its data, is what the caller can correlate the delivery on.
+    print(f"OBJECT_URL {created['url']}")
     return 0