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/domain/Big.Tests/AclHttpClientTests.cs

@@ -15,12 +15,13 @@ public class AclHttpClientTests
         var client = Client(capture.Responds(HttpStatusCode.OK,
             """{"zaakUrl":"http://openzaak/zaken/api/v1/zaken/abc"}"""));
 
-        var url = await client.OpenZaakAsync("123456782");
+        var url = await client.OpenZaakAsync("123456782", "reg-42");
 
         Assert.Equal("http://openzaak/zaken/api/v1/zaken/abc", url.ToString());
         Assert.Equal(HttpMethod.Post, capture.Seen!.Method);
         Assert.Equal("http://acl/zaken", capture.Seen.RequestUri!.ToString());
         Assert.Contains("\"bsn\":\"123456782\"", capture.Body);
+        Assert.Contains("\"reference\":\"reg-42\"", capture.Body);
     }
 
     [Fact]
@@ -29,7 +30,7 @@ public class AclHttpClientTests
         var capture = new RequestCapture();
         var client = Client(capture.Responds(HttpStatusCode.BadGateway));
 
-        await Assert.ThrowsAsync<HttpRequestException>(() => client.OpenZaakAsync("123456782"));
+        await Assert.ThrowsAsync<HttpRequestException>(() => client.OpenZaakAsync("123456782", "reg-1"));
     }
 
     [Fact]
@@ -38,7 +39,7 @@ public class AclHttpClientTests
         var capture = new RequestCapture();
         var client = Client(capture.Responds(HttpStatusCode.OK, "null"));
 
-        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => client.OpenZaakAsync("123456782"));
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => client.OpenZaakAsync("123456782", "reg-1"));
         Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase);
     }