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.IntegrationTests/OpenZaakGatewayIntegrationTests.cs

@@ -42,4 +42,32 @@ public sealed class OpenZaakGatewayIntegrationTests(OpenZaakFixture stack)
         Assert.Equal("517439943", zaak.GetProperty("bronorganisatie").GetString());
         Assert.Equal("openbaar", zaak.GetProperty("vertrouwelijkheidaanduiding").GetString());
     }
+
+    [Fact]
+    public async Task Setting_a_zaak_to_its_eindstatus_records_the_terminal_statustype()
+    {
+        var zaaktype = await stack.FindPublishedBigZaaktypeAsync();
+        Assert.True(zaaktype is not null,
+            "No published BIG-REGISTRATIE zaaktype found in OpenZaak — bring the stack up and " +
+            "seed it with OZ_PUBLISH=1 (`make integration` does this).");
+
+        var gateway = new OpenZaakGateway(stack.Http, stack.Options);
+        var zaakUrl = await gateway.OpenZaakAsync(new ZaakRequest(
+            Bronorganisatie: "517439943",
+            VerantwoordelijkeOrganisatie: "517439943",
+            Vertrouwelijkheidaanduiding: "openbaar",
+            Zaaktype: zaaktype!,
+            Startdatum: DateOnly.FromDateTime(DateTime.UtcNow)));
+
+        await gateway.SetZaakToEindstatusAsync(zaakUrl, zaaktype!, DateOnly.FromDateTime(DateTime.UtcNow));
+
+        // The zaak now carries a current status, and it is the zaaktype's eindstatus.
+        var zaak = await stack.GetZaakAsync(zaakUrl);
+        var statusUrl = zaak.GetProperty("status").GetString();
+        Assert.False(string.IsNullOrEmpty(statusUrl), "the approved zaak has no current status");
+
+        var status = await stack.GetJsonAsync(new Uri(statusUrl!));
+        var eindstatustype = await stack.FindEindstatustypeAsync(zaaktype!);
+        Assert.Equal(eindstatustype.ToString(), status.GetProperty("statustype").GetString());
+    }
 }