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.Api/Program.cs

@@ -20,7 +20,7 @@ app.MapGet("/health", () => "Healthy");
 // The ACL's single operation, exposed as a service endpoint.
 app.MapPost("/zaken", async (OpenZaakRequest body, AclService acl, CancellationToken ct) =>
 {
-    var zaakUrl = await acl.OpenZaakAsync(new DomainRegistration(body.Bsn), ct);
+    var zaakUrl = await acl.OpenZaakAsync(new DomainRegistration(body.Bsn, body.Reference), ct);
     return Results.Ok(new { zaakUrl = zaakUrl.ToString() });
 });
 
@@ -32,10 +32,20 @@ app.MapPost("/statussen", async (SetStatusRequest body, AclService acl, Cancella
     return Results.NoContent();
 });
 
+// Read a zaak's public-safe reference (its identificatie). The Event Subscriber calls this to enrich
+// the read projection without reading ZGW itself (§8.1, #78).
+app.MapPost("/zaken/reference", async (ZaakReferenceRequest body, AclService acl, CancellationToken ct) =>
+{
+    var reference = await acl.GetZaakReferenceAsync(new Uri(body.ZaakUrl), ct);
+    return Results.Ok(new { reference });
+});
+
 app.Run();
 
-public sealed record OpenZaakRequest(string Bsn);
+public sealed record OpenZaakRequest(string Bsn, string Reference);
 
 public sealed record SetStatusRequest(string ZaakUrl);
 
+public sealed record ZaakReferenceRequest(string ZaakUrl);
+
 public partial class Program;