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/acl/Acl.Application/AclService.cs

@@ -13,7 +13,8 @@ public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, ICloc
             defaults.VerantwoordelijkeOrganisatie,
             defaults.Vertrouwelijkheidaanduiding,
             defaults.ZaaktypeUrl,
-            clock.Today);
+            clock.Today,
+            registration.Reference);
 
         return gateway.OpenZaakAsync(request, ct);
     }
@@ -28,4 +29,12 @@ public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, ICloc
 
         return gateway.SetZaakToEindstatusAsync(zaakUrl, defaults.ZaaktypeUrl, clock.Today, ct);
     }
+
+    /// <summary>The zaak's reference (its ZGW identificatie), for the read projection (#78).</summary>
+    public Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+
+        return gateway.GetZaakIdentificatieAsync(zaakUrl, ct);
+    }
 }

services/acl/Acl.Application/DomainRegistration.cs

@@ -1,4 +1,5 @@
 namespace Acl.Application;
 
-/// <summary>Domain-language payload handed to the ACL. No ZGW concepts here.</summary>
-public sealed record DomainRegistration(string Bsn);
+/// <summary>Domain-language payload handed to the ACL. No ZGW concepts here. <see cref="Reference"/>
+/// is the registration's own id; the ACL records it as the zaak identificatie (adr-proposal #78).</summary>
+public sealed record DomainRegistration(string Bsn, string Reference);

services/acl/Acl.Application/IZaakGateway.cs

@@ -12,4 +12,8 @@ public interface IZaakGateway
     /// the catalogus and POSTs a status against the zaak, dated <paramref name="datumStatusGezet"/>.
     /// </summary>
     Task SetZaakToEindstatusAsync(Uri zaakUrl, Uri zaaktypeUrl, DateOnly datumStatusGezet, CancellationToken ct = default);
+
+    /// <summary>Read the zaak's <c>identificatie</c> — the public-safe reference the register shows.
+    /// The Event Subscriber calls this through the ACL rather than reading ZGW itself (§8.1, #78).</summary>
+    Task<string> GetZaakIdentificatieAsync(Uri zaakUrl, CancellationToken ct = default);
 }

services/acl/Acl.Application/ZaakRequest.cs

@@ -6,4 +6,5 @@ public sealed record ZaakRequest(
     string VerantwoordelijkeOrganisatie,
     string Vertrouwelijkheidaanduiding,
     Uri Zaaktype,
-    DateOnly Startdatum);
+    DateOnly Startdatum,
+    string Identificatie);