feat(acl): write the INGEDIEND record on submit and read records back (refs #153)

- OpenZaakAsync upserts a RegisterRecord with status INGEDIEND after opening the zaak,
  keyed on the same zaak id approval later upserts to INGESCHREVEN. The reference comes
  from the registration, so this path needs no ZGW read-back.
- ObjectenGateway.GetAsync fetches an object by the URL a notification carried — no
  objecttype resolution, no search — and reads 404 as "no record" rather than an error.
- POST /register-records/read exposes it to the Event Subscriber, which may not talk to
  Objecten itself (§8.1).

services/acl/Acl.Infrastructure/ObjectenGateway.cs

@@ -1,3 +1,4 @@
+using System.Net;
 using System.Net.Http.Headers;
 using System.Net.Http.Json;
 using System.Text.Json.Serialization;
@@ -38,8 +39,29 @@ public sealed class ObjectenGateway(HttpClient http, ObjectenOptions options, IC
                 "Updating the register record", ct);
     }
 
-    public Task<RegisterRecord?> GetAsync(Uri objectUrl, CancellationToken ct = default)
-        => throw new NotImplementedException();
+    public async Task<RegisterRecord?> GetAsync(Uri objectUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(objectUrl);
+
+        // Fetched by the URL the notification carried, so no objecttype resolution and no search —
+        // unlike a write, which has to find the object for a registration id.
+        using var message = new HttpRequestMessage(HttpMethod.Get, objectUrl);
+        message.Headers.Authorization = new AuthenticationHeaderValue("Token", options.Token);
+        message.Headers.Add("Accept-Crs", "EPSG:4326");
+
+        using var response = await http.SendAsync(message, ct);
+        // The object may be gone by the time a (possibly redelivered) notification is handled —
+        // there is simply nothing to project, which is not a failure (§8.6).
+        if (response.StatusCode == HttpStatusCode.NotFound)
+            return null;
+
+        await EnsureSuccessAsync(response, "Reading the register record", ct);
+
+        var body = await response.Content.ReadFromJsonAsync<ReadObjectDto>(ct)
+            ?? throw new InvalidOperationException("Objecten returned an empty object response");
+        var data = body.Record?.Data;
+        return data is null ? null : new RegisterRecord(data.Id, data.Status, data.Reference);
+    }
 
     private RecordDto NewRecord(int typeVersion, RecordDataDto data) =>
         new(typeVersion, data, clock.Today.ToString("yyyy-MM-dd"));
@@ -144,6 +166,12 @@ public sealed class ObjectenGateway(HttpClient http, ObjectenOptions options, IC
     private sealed record ObjectDto(
         [property: JsonPropertyName("url")] string Url);
 
+    private sealed record ReadObjectDto(
+        [property: JsonPropertyName("record")] ReadRecordDto? Record);
+
+    private sealed record ReadRecordDto(
+        [property: JsonPropertyName("data")] RecordDataDto? Data);
+
     private sealed record CreateObjectDto(
         [property: JsonPropertyName("type")] string Type,
         [property: JsonPropertyName("record")] RecordDto Record);