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.Tests/OpenZaakGatewayTests.cs

@@ -22,7 +22,7 @@ public class OpenZaakGatewayTests
 
     private static ZaakRequest SampleRequest() => new(
         "517439943", "517439943", "openbaar",
-        new("http://openzaak/catalogi/api/v1/zaaktypen/big"), new DateOnly(2026, 6, 4));
+        new("http://openzaak/catalogi/api/v1/zaaktypen/big"), new DateOnly(2026, 6, 4), "REG-REF-1");
 
     private static StubHandler Created(out RequestCapture capture)
     {
@@ -67,6 +67,8 @@ public class OpenZaakGatewayTests
         Assert.Contains("\"vertrouwelijkheidaanduiding\":\"openbaar\"", capture.Body);
         Assert.Contains("\"startdatum\":\"2026-06-04\"", capture.Body);
         Assert.Contains("\"zaaktype\":\"http://openzaak/catalogi/api/v1/zaaktypen/big\"", capture.Body);
+        // The registration reference is set as the zaak identificatie (#78).
+        Assert.Contains("\"identificatie\":\"REG-REF-1\"", capture.Body);
     }
 
     [Fact]
@@ -357,6 +359,61 @@ public class OpenZaakGatewayTests
         Assert.Equal(4, rec.Requests.Count);
     }
 
+    [Fact]
+    public async Task Reading_a_zaak_returns_its_identificatie_with_bearer_and_crs()
+    {
+        RequestCapture? capture = null;
+        var handler = new StubHandler(req =>
+        {
+            capture = new RequestCapture { Seen = req };
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = JsonContent.Create(new { identificatie = "REG-XYZ", url = ZaakUrl }),
+            });
+        });
+
+        var reference = await Gateway(handler).GetZaakIdentificatieAsync(new Uri(ZaakUrl));
+
+        Assert.Equal("REG-XYZ", reference);
+        Assert.Equal(HttpMethod.Get, capture!.Seen!.Method);
+        Assert.Equal(ZaakUrl, capture.Seen.RequestUri!.ToString());
+        Assert.Equal("Bearer", capture.Seen.Headers.Authorization!.Scheme);
+        Assert.Equal("EPSG:4326", Assert.Single(capture.Seen.Headers.GetValues("Accept-Crs")));
+    }
+
+    [Fact]
+    public async Task Reading_a_zaak_throws_when_openzaak_rejects_it()
+    {
+        var handler = new StubHandler(_ =>
+            Task.FromResult(new HttpResponseMessage(HttpStatusCode.NotFound) { Content = new StringContent("nope") }));
+
+        var ex = await Assert.ThrowsAsync<HttpRequestException>(
+            () => Gateway(handler).GetZaakIdentificatieAsync(new Uri(ZaakUrl)));
+        Assert.Contains("Reading the zaak", ex.Message);
+    }
+
+    [Fact]
+    public async Task Reading_a_zaak_throws_when_openzaak_returns_an_empty_body()
+    {
+        var handler = new StubHandler(_ =>
+            Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent("null", System.Text.Encoding.UTF8, "application/json"),
+            }));
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => Gateway(handler).GetZaakIdentificatieAsync(new Uri(ZaakUrl)));
+        Assert.Contains("empty zaak response", ex.Message);
+    }
+
+    [Fact]
+    public async Task Reading_a_null_zaak_is_rejected()
+    {
+        var handler = new StubHandler(_ => throw new InvalidOperationException("should not be sent"));
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => Gateway(handler).GetZaakIdentificatieAsync(null!));
+    }
+
     [Fact]
     public async Task Approving_rejects_a_null_zaak_or_zaaktype()
     {