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/AclService.cs

@@ -17,4 +17,15 @@ public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, ICloc
 
         return gateway.OpenZaakAsync(request, ct);
     }
+
+    /// <summary>
+    /// Approve a zaak: set it to the eindstatus of the configured BIG zaaktype (ADR-0003 default). The
+    /// domain hands over only the zaak URL; the ACL owns which statustype means "approved" (§8.1).
+    /// </summary>
+    public Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+
+        return gateway.SetZaakToEindstatusAsync(zaakUrl, defaults.ZaaktypeUrl, clock.Today, ct);
+    }
 }