feat(domain): ApproveRegistration use case + temp /registrations/{id}/approve endpoint (refs #75)

Loads the registration, asks the ACL to set its zaak's final status (new
IAclClient.ApproveZaakAsync → POST /statussen), then advances the aggregate to
INGESCHREVEN. Idempotent: a repeated approval is a no-op. Adds HTTP-adapter tests.

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

services/domain/Big.Application/Ports.cs

@@ -25,6 +25,12 @@ public interface IWorkflowClient
 public interface IAclClient
 {
     Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default);
+
+    /// <summary>
+    /// Record the approval on the given zaak. The ACL translates this to the ZGW concept — setting
+    /// the zaak's final status — which OpenZaak notifies over NRC; the domain never names statustypen.
+    /// </summary>
+    Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default);
 }
 
 /// <summary>