feat(domain): implement SubmitRegistration and OpenZaakWorker (refs #6)

SubmitRegistration creates the aggregate, persists it, starts the registratie
process via the Workflow Client, records the instance id and upserts. OpenZaakWorker
loads the correlated registration, opens a zaak via the ACL, attaches it and saves;
an unknown registration throws (job redelivered), and an already-opened zaak short-
circuits without opening a second one (§8.6).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Application/OpenZaakWorker.cs

@@ -20,8 +20,17 @@ public sealed class OpenZaakWorker(IRegistrationStore store, IAclClient acl)
     {
         ArgumentNullException.ThrowIfNull(job);
 
-        // STUB (red): does not load, call the ACL, or attach.
-        await Task.CompletedTask;
-        return new Uri("about:blank");
+        var registration = await store.GetAsync(job.RegistrationId, ct)
+            ?? throw new InvalidOperationException(
+                $"No registration {job.RegistrationId} for OpenZaakAanmaken job {job.JobId}.");
+
+        // A redelivered job whose zaak was already opened completes without opening a second one.
+        if (registration.ZaakUrl is not null)
+            return registration.ZaakUrl;
+
+        var zaakUrl = await acl.OpenZaakAsync(registration.Bsn, ct);
+        registration.AttachZaak(zaakUrl);
+        await store.SaveAsync(registration, ct);
+        return zaakUrl;
     }
 }