feat(infra): Objecten publishes register events to NRC (closes #152) (#154)

## What & why

S-19b-1. A write to the Objecten API now produces a **delivered** notification on the
`objecten` kanaal in Open Notificaties. ADR-0028 switched Objecten's notifications off on
purpose — there was no broker, worker, kanaal or abonnement, so wiring only the client side
would have dropped every message on the floor. This slice builds the real path and turns it
back on.

- `objecten-celery` worker (mirrors `oz-celery`) + `CELERY_BROKER_URL`/`RESULT_BACKEND` on
  objecten-redis db 1 (db 0 is already the cache). `notifications_api_common` only *queues*
  the send; without a worker every register write is silently undelivered.
- `nrc` service + `notifications_config` in Objecten's `setup_configuration`, reusing the
  `big-reference-seed` credential OpenZaak publishes with (NRC authorizes it via OpenZaak's
  AC, which grants it `heeft_alle_autorisaties` — no second credential needed).
- The `objecten` kanaal in NRC's `setup_configuration`. The name is fixed by the Objects API
  (`NOTIFICATIONS_KANAAL`), not chosen here; publishing to an unregistered kanaal is exactly
  what the red check reported first.
- `NOTIFICATIONS_DISABLED: "false"` in both compose files.
- Writers address Objecten as `objecten.local` — see *Notes for reviewers*.
- `make verify-objecten-notifications` — registers an abonnement on `objecten` pointing at a
  throwaway sink, writes a `RegisterRecord` exactly as the ACL does on approval, asserts the
  delivery. One assertion covering the whole chain: Objecten -> objecten-celery -> NRC ->
  nrc-beat -> callback. Wired into the CI `verify-stack` job and the summary table.

**ADR-0029** records the decisions; ADR-0028's ceiling now points at it.

Closes #152

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the implementation (dc9ca2c, red at the first hop:
      `NRC POST /api/v1/abonnement -> 400 "Kanaal met deze naam bestaat niet."`).
- [x] Implementation makes the test pass (4488962, + two fixes found by CI, below).
- [x] Conventional Commits referencing the issue (`refs #152`).
- [x] CI green — all six jobs on a5fd47e, including `verify-stack` end to end (e2e included).
- [x] `docker compose up` from a fresh clone reaches green health checks within 3 minutes
      (`verify-stack`'s bring-up step).
- [x] Docs updated — ADR-0029 added, ADR-0028's ceiling annotated, BACKLOG.md split.
- [x] ADR added in `docs/architecture/`.
- [x] Demo note in `docs/demo-script.md` if user-visible — n/a, infrastructure only; nothing
      consumes the kanaal until S-19b-2 (#153).

## Notes for reviewers

**The one genuinely non-obvious bit: writers address Objecten as `objecten.local:8000`, not
`objecten:8000`.** NRC types a notification's `hoofdObject`/`resourceUrl` as DRF `URLField`,
so Django's `URLValidator` runs on them — and it rejects a **single-label** host. Objecten
fills both from the object url DRF built with `request.build_absolute_uri`, i.e. *the Host
the caller used*. Writing via the plain service name returns 201 and then fails every
publish in the background, forever, with

```
400 {"hoofdObject":["Voer een geldige URL in."],"resourceUrl":["Voer een geldige URL in."]}
```

So the `objecten` service carries an `objecten.local` network alias and every writer uses it
— `Acl__Objecten__BaseUrl`, `ObjectenGatewayIntegrationTests`, this slice's verify driver.
An alias rather than a bare dotted `SITE_DOMAIN` so the host still *resolves*: a subscriber
following `resourceUrl` reaches the record, which S-19b-2 will do. Readers keep the plain
name. Same class of constraint as ADR-0028's Objecttypen base-URL rule.

**Ceiling, stated in the ADR:** nothing enforces the alias — a future writer using
`objecten:8000` gets a 201 and silently no notification. If a second writer ever appears,
rename the compose service rather than adding a lint.

**Two CI-only failures on the way here**, both worth knowing:
1. `SITE_DOMAIN` was my first guess at the mechanism and is simply not what builds those
   URLs — dropped in d76abf2.
2. The check correlated the delivery on the `reference` inside the record it wrote. An NRC
   notification carries `kanaal`/`resource`/`kenmerken`/`hoofdObject`/`resourceUrl` and
   **never the record data**, so it correlates on the object URL now (a5fd47e).

**Cost:** one more long-running container on the memory-tight runner. It inherits the capped
`UWSGI_PROCESSES: "1"` env, which the celery command ignores; if `verify-stack` gets tight
again, celery concurrency is the next knob.

**Follow-up:** S-19b-2 (#153) sources the projection from these events. Nothing subscribes to
the `objecten` kanaal in the product yet — only the verify check does.Reviewed-on: #154

Makefile

@@ -43,7 +43,7 @@ export DOCKER_HOST := unix://$(PODMAN_SOCK)
 endif
 endif
 
-.PHONY: ci lint build unit mutation frontend integration verify verify-up verify-acl verify-nrc verify-projection verify-bff verify-domain verify-observability verify-tracing verify-metrics verify-objecttypen verify-objecten verify-registerrecord verify-notifications smoke up down local verify-local local-down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down flowable-up flowable-smoke flowable-down help
+.PHONY: ci lint build unit mutation frontend integration verify verify-up verify-acl verify-nrc verify-projection verify-bff verify-domain verify-observability verify-tracing verify-metrics verify-objecttypen verify-objecten verify-registerrecord verify-objecten-notifications verify-notifications smoke up down local verify-local local-down changelog openzaak-up openzaak-smoke openzaak-seed openzaak-down stack-up stack-smoke stack-down keycloak-up keycloak-smoke keycloak-down flowable-up flowable-smoke flowable-down help
 
 ## ci: run the full pipeline — lint, build, unit, mutation, frontend, verify (mirrors Gitea Actions)
 ## `verify` is the live-stack stage (full stack up once → ACL + notification checks).
@@ -201,6 +201,11 @@ verify-objecten:
 verify-registerrecord:
 	bash infra/run-registerrecord-check.sh
 
+## verify-objecten-notifications: assert a RegisterRecord write in Objecten is DELIVERED as an
+## `objecten` notification via NRC (S-19b-1), against the already-running stack.
+verify-objecten-notifications:
+	bash infra/run-objecten-notifications-check.sh
+
 ## verify: local mirror of the CI verify-stack job — full stack up once, all checks,
 ## tear down (always). For fast single-concern local iteration use `integration`
 ## (oz-only) or `verify-notifications` (oz+nrc) instead.
@@ -212,6 +217,7 @@ verify:
 	    && bash infra/run-acl-integration.sh \
 	    && bash infra/run-notification-check.sh \
 	    && bash infra/run-projection-check.sh \
+	    && bash infra/run-objecten-notifications-check.sh \
 	    && bash infra/run-domain-check.sh \
 	    && bash infra/run-bff-check.sh \
 	    && bash infra/run-e2e-check.sh || rc=$$?; \