feat(acl): set a zaak to its eindstatus via ZGW + POST /statussen endpoint (refs #75)

The gateway resolves the zaaktype's eindstatus from the catalogus (isEindstatus,
falling back to the highest volgnummer) and POSTs a status against the zaak. Exposed
as POST /statussen for the domain's approve use case. Adds an integration test that
sets the eindstatus against a real OpenZaak and verifies the zaak's current status.

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

services/acl/Acl.Application/IZaakGateway.cs

@@ -5,4 +5,11 @@ namespace Acl.Application;
 public interface IZaakGateway
 {
     Task<Uri> OpenZaakAsync(ZaakRequest request, CancellationToken ct = default);
+
+    /// <summary>
+    /// Set the given zaak to the <em>eindstatus</em> (final statustype) of the supplied zaaktype —
+    /// the ZGW translation of "approve". The gateway resolves which statustype is the eindstatus from
+    /// the catalogus and POSTs a status against the zaak, dated <paramref name="datumStatusGezet"/>.
+    /// </summary>
+    Task SetZaakToEindstatusAsync(Uri zaakUrl, Uri zaaktypeUrl, DateOnly datumStatusGezet, CancellationToken ct = default);
 }