feat(#78): one citizen reference across self-service and the openbaar register (#79)

## What & why

Before this change the self-service confirmation and the openbaar register showed **different** identifiers, so a citizen could not look their registration back up (#78). Now both surface the same **reference**:

- **domain → ACL (write):** the domain `registrationId` is set as the zaak's `identificatie` on `POST /zaken`.
- **event-subscriber → ACL (read):** the subscriber reads the zaak's `identificatie` back through the ACL (§8.1 — only the ACL talks to ZGW) via a new `POST /zaken/reference`, and stores it on the projection row **and** the `processed_notifications` replay log.
- **BFF + openbaar:** the public view exposes `id/status/reference` (never bsn/naam) and searches by id or reference; the register's "Referentie" column shows the reference.

Storing the reference in the replay log keeps ADR-0008's **rebuild-is-log-only** invariant intact — `/admin/rebuild` reproduces the reference without re-reading the ACL.

Decision recorded in **ADR-0012**.

## Definition of Done

- [x] Linked issue: #78
- [x] Tests written first; red → green per layer
- [x] Unit + acceptance green (`make unit`): domain 49, acl 27, bff 20, event-subscriber 19, acceptance 7
- [x] Frontend lint + test green (`nx run-many -t lint test`)
- [x] Mutation ≥ break(90): acl 100%, event-subscriber 100%, bff 100%, domain 98.41% (pre-existing FlowableWorkflowClient baseline, untouched)
- [x] e2e extended: confirmation reference == register reference
- [x] openapi.json + api-client regenerated (drift guard green)
- [x] ADR-0012 added; demo-script note appended
- [x] `Acl__BaseUrl` wired for the subscriber in compose

closes #78

Reviewed-on: #79

services/event-subscriber/EventSubscriber.Application/Notification.cs

@@ -24,10 +24,12 @@ public sealed record Notification(
     public bool IsZaakStatusSet =>
         Kanaal == "zaken" && Resource == "status" && Actie == "create";
 
-    /// <summary>The zaak UUID used as the projection key. For a status notification the resource URL is
-    /// the status, so the zaak comes from <c>hoofdObject</c> (which, for a zaak-create, equals the
-    /// resource URL) — see the NRC payload note.</summary>
-    public string ZaakId => (HoofdObject ?? ResourceUrl).Segments[^1].Trim('/');
+    /// <summary>The zaak URL this notification concerns — <c>hoofdObject</c> (the zaak) for a status
+    /// notification, else the resource URL (which, for a zaak-create, is the zaak).</summary>
+    public Uri ZaakUrl => HoofdObject ?? ResourceUrl;
+
+    /// <summary>The zaak UUID used as the projection key — the trailing segment of <see cref="ZaakUrl"/>.</summary>
+    public string ZaakId => ZaakUrl.Segments[^1].Trim('/');
 
     /// <summary>
     /// A deterministic dedup key. Open Notificaties carries no notification id and may