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.Tests/AclHttpClientTests.cs

@@ -41,4 +41,27 @@ public class AclHttpClientTests
         var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => client.OpenZaakAsync("123456782"));
         Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase);
     }
+
+    [Fact]
+    public async Task Approves_a_zaak_by_posting_its_url_to_statussen()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.NoContent));
+
+        await client.ApproveZaakAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+
+        Assert.Equal(HttpMethod.Post, capture.Seen!.Method);
+        Assert.Equal("http://acl/statussen", capture.Seen.RequestUri!.ToString());
+        Assert.Contains("\"zaakUrl\":\"http://openzaak/zaken/api/v1/zaken/abc\"", capture.Body);
+    }
+
+    [Fact]
+    public async Task Approve_throws_when_the_acl_rejects_the_request()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.BadGateway));
+
+        await Assert.ThrowsAsync<HttpRequestException>(
+            () => client.ApproveZaakAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc")));
+    }
 }