feat: read projection sourced from the register in Objecten (closes #153) (#155)

## What & why

S-19b-2, closing out ADR-0028's stated direction: **the read projection is now derived from the
`RegisterRecord` in Objecten, not from ZGW zaak events.**

Until now the subscriber listened on `zaken` and *inferred* register state from case events — a
`zaak/create` meant INGEDIEND, and any `status/create` was assumed to be the approval (it may not
read OpenZaak, so it could not tell statustypen apart). The reference wasn't in the notification
at all, so every projection made a second hop to the ACL. The register — a fact about a person —
was being reconstructed by guessing at the lifecycle of the case that produced it.

- The subscriber's abonnement moves to the `objecten` kanaal (S-19b-1 made it publish).
- An Objecten notification carries **no record data**, only the object URL, so the record is read
  back through the ACL (`POST /register-records/read`) — §8.1 applies to Objecten exactly as
  ADR-0028 established.
- The record carries `id`, `status` and `reference`, so the row *is* the record: `IsZaakCreated`,
  `IsZaakStatusSet`, `ZaakUrl`, `ZaakId` and `ToEntry`'s `Resource == "status"` inference are all
  gone, and so is the ACL enrichment hop.
- **The ACL now writes an INGEDIEND record on submit.** Without it, re-sourcing would silently
  drop every submitted registration from the public register, since only approval wrote a record.
- `processed_notifications` holds the projected row (`register_id`, `status`, `reference`) instead
  of the ZGW event, so a rebuild is a replay with no mapping rules and no upstream reads at all.

**ADR-0030** records it. ADR-0028's open caveat — record written but not yet read, "the two must
agree" — is closed: there is one source now.

Closes #153

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing tests committed before the implementation — two red/green pairs, ACL side
      (06c0444 → 566ef7d) and subscriber side (142ed45 → 8af09b2).
- [x] Refactor commit follows (b496ac9).
- [x] Conventional Commits referencing the issue (`refs #153`).
- [x] CI green — all six jobs on b30fa66, `verify-stack` end to end including the e2e.
- [x] `docker compose up` from a fresh clone reaches green health checks within 3 minutes
      (`verify-stack`'s bring-up step — see the wait-healthy fix below).
- [x] Docs updated — ADR-0030 added, ADR-0028's consequence + caveat annotated, BACKLOG.md,
      e2e header comment.
- [x] ADR added in `docs/architecture/`.
- [x] Demo note in `docs/demo-script.md` — n/a: no user-visible change. The openbaar register
      shows the same two statuses for the same registrations; only where they come from changed.

## Notes for reviewers

**The decision I'd most like a second opinion on** is the one the issue didn't settle: what
happens to INGEDIEND. Objecten held only INGESCHREVEN records, so re-sourcing forced a choice
between (a) the ACL also writing on submit, (b) a public register that lists only actual
registrations, or (c) a hybrid keeping both kanalen. I took (a): visible behaviour is unchanged
and the register holds the whole lifecycle. (b) is arguably the better *semantics* for a public
register but narrows what the portal shows and reads against PRD §68 ("~50 register entries with
diverse statuses"); (c) leaves the projection half-derived from ZGW, which is the coupling
ADR-0028 set out to remove. All three are laid out in ADR-0030.

**The dedup key is the projected row**, `objecten:object:{url}:{status}:{reference}` — not the
object URL (the ACL upserts *one object per registration*, so submit and approval notify about
the same URL and the approval would be swallowed as a duplicate) and not URL+actie (a retried
approval is a second `update`). Redeliveries collapse, genuine state changes don't. §8.6.

**The migration drops columns rather than renaming them.** EF scaffolded renames — `resource` →
`register_id`, `zaak_id` → `status` — which would have carried ZGW values into columns meaning
something else, and a rebuild would then have projected that garbage. It also empties both
tables: a pre-slice row describes a zaak event the new projector can't reproject, and those
registrations have no RegisterRecord in Objecten either, so they're not re-derivable from the new
source. Stated as a ceiling in the ADR — fine while stacks are ephemeral, backfill from Objecten
if a long-lived environment ever needs it.

**`run-projection-check.sh` now opens its zaak through the ACL** instead of straight against
OpenZaak, because the ACL is what writes the record. A zaak created behind the ACL's back
produces no projection row — that's the re-source working, not a gap.

## Three fixes CI found, none of them in the projection logic

1. **`wait-healthy.sh` matched the wrong container** (744f91a). Bring-up timed out with
   `TIMEOUT: 'objecten' not healthy (status=none)` while the `docker ps` it dumps showed
   objecten `Up 9 minutes (healthy)`. `--filter name=` is a substring match, so `objecten` also
   matches `objecten-db`/`objecten-redis`/`objecten-celery`, and `head -1` took whichever docker
   listed first — the celery worker has no healthcheck, hence `status=none`. Latent since those
   services landed and decided purely by listing order; `objecttypen` matches `objecttypen-db`
   the same way. Anchored on the compose replica suffix, which the verify scripts already do.
2. **The ACL had to be repointed at OpenZaak's IP** (7e0897a). Opening the zaak through the ACL
   put this check in the same bind run-domain-check.sh already handles:
   `400 {"name":"zaaktype","code":"bad-url","reason":"Voer een geldige URL in."}`. OpenZaak
   reflects the request Host into the zaaktype URL and then rejects it on zaak-create when
   single-label — the mechanism compose already documents on `ACL_OPENZAAK_BASEURL`.
3. **Approval arrives as `partial_update`, not `update`** (0dd26a7 → b30fa66) — the one real bug
   in the slice. The ACL upserts with PATCH; DRF routes it through the notifying `update()` but
   names the action `partial_update`, so the projector dropped every approval. Only the e2e could
   catch it: `verify-projection` drives a submit, and per ADR-0028 the e2e is the only check that
   drives a *real* approval.

`verify-tracing` also failed once (run 722) on a path this PR doesn't touch, and passed on a
plain re-run of the same commit. Tempo logged `pusher failed to consume trace data` /
`distributor_pool failing healthcheck` — it dropped spans under runner load rather than the trace
chain being broken. Filed as **#156** rather than absorbed here.

**Correction to the #152 PR notes:** I wrote there that celery concurrency was "the next knob" if
verify-stack got tight. It isn't — `CELERY_WORKER_CONCURRENCY` already defaults to 1 in the Maykin
image, so `objecten-celery` is already a single-process worker. Noted in #156.

**Possible follow-up, deliberately not done here:** an `openzaak.local` network alias mirroring
`objecten.local` would remove the ACL-repoint dance from both run-domain-check.sh and
run-projection-check.sh. It changes the host in every zaak URL the system produces, which is too
broad a ripple to land inside an unrelated slice — worth its own issue.

**Known costs, all in the ADR:** submission is now two writes across two modules and eventually
consistent (same posture ADR-0028 accepted for approval); projecting now depends on the ACL being
reachable on the main path, not just for enrichment (NRC retries, so it converges); and OpenZaak
still publishes to `zaken` with nothing in the product listening — kept because `verify-nrc`
asserts that path.Reviewed-on: #155

services/acl/Acl.Infrastructure/ObjectenGateway.cs

@@ -1,3 +1,4 @@
+using System.Net;
 using System.Net.Http.Headers;
 using System.Net.Http.Json;
 using System.Text.Json.Serialization;
@@ -38,6 +39,30 @@ public sealed class ObjectenGateway(HttpClient http, ObjectenOptions options, IC
                 "Updating the register record", ct);
     }
 
+    public async Task<RegisterRecord?> GetAsync(Uri objectUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(objectUrl);
+
+        // Fetched by the URL the notification carried, so no objecttype resolution and no search —
+        // unlike a write, which has to find the object for a registration id.
+        using var message = new HttpRequestMessage(HttpMethod.Get, objectUrl);
+        message.Headers.Authorization = new AuthenticationHeaderValue("Token", options.Token);
+        message.Headers.Add("Accept-Crs", "EPSG:4326");
+
+        using var response = await http.SendAsync(message, ct);
+        // The object may be gone by the time a (possibly redelivered) notification is handled —
+        // there is simply nothing to project, which is not a failure (§8.6).
+        if (response.StatusCode == HttpStatusCode.NotFound)
+            return null;
+
+        await EnsureSuccessAsync(response, "Reading the register record", ct);
+
+        var body = await response.Content.ReadFromJsonAsync<ReadObjectDto>(ct)
+            ?? throw new InvalidOperationException("Objecten returned an empty object response");
+        var data = body.Record?.Data;
+        return data is null ? null : new RegisterRecord(data.Id, data.Status, data.Reference);
+    }
+
     private RecordDto NewRecord(int typeVersion, RecordDataDto data) =>
         new(typeVersion, data, clock.Today.ToString("yyyy-MM-dd"));
 
@@ -141,6 +166,12 @@ public sealed class ObjectenGateway(HttpClient http, ObjectenOptions options, IC
     private sealed record ObjectDto(
         [property: JsonPropertyName("url")] string Url);
 
+    private sealed record ReadObjectDto(
+        [property: JsonPropertyName("record")] ReadRecordDto? Record);
+
+    private sealed record ReadRecordDto(
+        [property: JsonPropertyName("data")] RecordDataDto? Data);
+
     private sealed record CreateObjectDto(
         [property: JsonPropertyName("type")] string Type,
         [property: JsonPropertyName("record")] RecordDto Record);