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/SubmitRegistration.cs

@@ -18,8 +18,16 @@ public sealed class SubmitRegistration(IRegistrationStore store, IWorkflowClient
     {
         ArgumentNullException.ThrowIfNull(command);
 
-        // STUB (red): no persistence, no process start.
-        await Task.CompletedTask;
-        return RegistrationId.New();
+        var registration = Registration.Submit(command.Bsn);
+
+        // Persist before starting the process so the worker can correlate the OpenZaakAanmaken
+        // job back to an aggregate that already exists (ADR-0009).
+        await store.SaveAsync(registration, ct);
+
+        var processInstanceId = await workflow.StartRegistrationProcessAsync(registration.Id, ct);
+        registration.RecordProcessStarted(processInstanceId);
+        await store.SaveAsync(registration, ct);
+
+        return registration.Id;
     }
 }