feat(acl): write the RegisterRecord to Objecten on approval (refs #149)

ApproveZaakAsync now does two writes: the ZGW eindstatus (the process) and the
register record in Objecten (the register). The record is keyed on the zaak
UUID — the same key the read projection rows carry — and its reference is the
zaak's identificatie, so nothing personal crosses into the world-readable
register (ADR-0027).

ObjectenGateway resolves the objecttype by name (its URL and version are
assigned at seed time, as with ADR-0021), searches for an existing object by
data attribute, then POSTs or PATCHes. Resolution is lazy, so the ACL needs no
depends_on on Objecten and does not crash-loop when it boots first.

services/acl/Acl.Application/AclService.cs

@@ -28,16 +28,33 @@ public sealed class AclService(
     }
 
     /// <summary>
-    /// Approve a zaak: set it to the eindstatus of the BIG zaaktype (resolved by identificatie, S-27).
-    /// The domain hands over only the zaak URL; the ACL owns which statustype means "approved" (§8.1).
+    /// Approve a zaak: set it to the eindstatus of the BIG zaaktype (resolved by identificatie, S-27),
+    /// then write the register record to Objecten (S-19a). The domain hands over only the zaak URL; the
+    /// ACL owns which statustype means "approved" and what the register record looks like (§8.1).
     /// </summary>
+    /// <remarks>
+    /// OpenZaak holds the process, Objecten holds the register (ADR-0028), so approval is two writes
+    /// across two modules and is eventually consistent by construction. Both are idempotent — a status
+    /// is a log entry, the record upsert is keyed on the zaak id — so a caller that retries a failed
+    /// approval converges rather than duplicating.
+    /// </remarks>
     public async Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default)
     {
         ArgumentNullException.ThrowIfNull(zaakUrl);
 
         await gateway.SetZaakToEindstatusAsync(zaakUrl, await catalog.GetZaaktypeUrlAsync(ct), clock.Today, ct);
+
+        await register.UpsertAsync(
+            new RegisterRecord(
+                ZaakId(zaakUrl),
+                RegisterRecordStatus.Ingeschreven,
+                await gateway.GetZaakIdentificatieAsync(zaakUrl, ct)),
+            ct);
     }
 
+    /// <summary>The zaak's UUID — the key the register record and the read projection rows share.</summary>
+    private static string ZaakId(Uri zaakUrl) => zaakUrl.Segments[^1].TrimEnd('/');
+
     /// <summary>
     /// Cancel a zaak on document-timeout expiry (S-10c): set it to the BIG zaaktype's cancellation
     /// statustype + resultaat. The domain hands over only the zaak URL; the ACL owns which