test(infra): verify-objecten-notifications asserts Objecten publishes to NRC (refs #152)

Registers an abonnement on the `objecten` kanaal pointing at the existing
webhook sink, writes a RegisterRecord exactly as the ACL does on approval, and
waits for the delivery. That is the whole publish chain in one assertion:
Objecten → its celery worker → NRC → nrc-beat → the callback.

Fails today at the first hop, which is the point:

  NRC POST /api/v1/abonnement → 400: {"name":"naam","code":"kanaal_naam",
  "reason":"Kanaal met deze naam bestaat niet."}

Also splits S-19b (#150) into #152/#153 in BACKLOG.md — the notification wiring
and the projection re-source are independently deployable (CLAUDE.md §13).

BACKLOG.md

@@ -296,7 +296,9 @@ Split into independently deployable sub-slices (CLAUDE.md §13):
 Split into independently deployable sub-slices (CLAUDE.md §13):
 
 - **S-19a** (#149, ✅) · ACL writes the `RegisterRecord` to Objecten on approval, idempotently, alongside the ZGW eindstatus. Carries the ADR (ADR-0028).
-- **S-19b** (#150) · Read projection sourced from Objecten instead of NRC zaak events. Depends on S-19a.
+- **S-19b** (#150) · Read projection sourced from Objecten instead of NRC zaak events. *(split — #150 closed)*
+  - **S-19b-1** (#152) · Objecten publishes to NRC — broker, celery worker, `objecten` kanaal, notifications config. Turns back on what ADR-0028 deliberately disabled.
+  - **S-19b-2** (#153) · Projection derived from `RegisterRecord` objects, rebuildable from the Objecten-derived log. Depends on S-19b-1.
 
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