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.Api/Program.cs

@@ -24,8 +24,18 @@ app.MapPost("/zaken", async (OpenZaakRequest body, AclService acl, CancellationT
     return Results.Ok(new { zaakUrl = zaakUrl.ToString() });
 });
 
+// Approve a zaak: set it to its zaaktype's eindstatus (S-09b). The domain hands over only the zaak
+// URL; the ACL owns the ZGW statustype resolution (§8.1).
+app.MapPost("/statussen", async (SetStatusRequest body, AclService acl, CancellationToken ct) =>
+{
+    await acl.ApproveZaakAsync(new Uri(body.ZaakUrl), ct);
+    return Results.NoContent();
+});
+
 app.Run();
 
 public sealed record OpenZaakRequest(string Bsn);
 
+public sealed record SetStatusRequest(string ZaakUrl);
+
 public partial class Program;