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.Api/AclHttpClient.cs

@@ -0,0 +1,30 @@
+using System.Net.Http.Json;
+using System.Text.Json.Serialization;
+using EventSubscriber.Application;
+
+namespace EventSubscriber.Api;
+
+/// <summary>
+/// HTTP client to the ACL service. The subscriber enriches the projection with the zaak's reference
+/// (identificatie) by asking the ACL — the only code that may read ZGW (§8.1) — rather than reading
+/// OpenZaak itself (adr-proposal #78).
+/// </summary>
+public sealed class AclHttpClient(HttpClient http) : IAclClient
+{
+    public async Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+
+        using var response = await http.PostAsJsonAsync(
+            new Uri(http.BaseAddress!, "zaken/reference"), new ReferenceRequest(zaakUrl.ToString()), ct);
+        response.EnsureSuccessStatusCode();
+
+        var body = await response.Content.ReadFromJsonAsync<ReferenceResponse>(ct)
+            ?? throw new InvalidOperationException("The ACL returned an empty reference response.");
+        return body.Reference;
+    }
+
+    private sealed record ReferenceRequest([property: JsonPropertyName("zaakUrl")] string ZaakUrl);
+
+    private sealed record ReferenceResponse([property: JsonPropertyName("reference")] string Reference);
+}

services/event-subscriber/EventSubscriber.Api/Program.cs

@@ -11,9 +11,15 @@ var connectionString = builder.Configuration.GetConnectionString("Projection")
 // this value (ADR-0007), so the webhook enforces it.
 var webhookToken = builder.Configuration["EventSubscriber:Webhook:AuthToken"]
     ?? throw new InvalidOperationException("Missing configuration 'EventSubscriber:Webhook:AuthToken'");
+// The ACL is the only code that may read ZGW (§8.1); the subscriber enriches the projection with the
+// zaak reference through it (adr-proposal #78).
+var aclBaseUrl = builder.Configuration["Acl:BaseUrl"]
+    ?? throw new InvalidOperationException("Missing configuration 'Acl:BaseUrl'");
 
 builder.Services.AddProjectionReadModel(connectionString);
 builder.Services.AddProjectionWriteSide();
+builder.Services.AddHttpClient<EventSubscriber.Application.IAclClient, EventSubscriber.Api.AclHttpClient>(
+    c => c.BaseAddress = new Uri(aclBaseUrl));
 
 var app = builder.Build();