test(event-subscriber): the projection is sourced from register records (refs #153)

Ports, schema and failing tests for the subscriber half of S-19b-2, ahead of the
implementation.

The subscriber now listens on the `objecten` kanaal instead of `zaken`. An Objecten
notification carries no record data — only the object URL — so the record is read back
through the ACL (§8.1), and the zaak-shaped surface goes away: IsZaakCreated /
IsZaakStatusSet / ZaakUrl / ZaakId and ToEntry's `Resource == "status"` mapping are replaced
by IsRegisterRecordWritten + ObjectUrl.

The notification log now holds the projected row itself (register id, status, reference),
so a rebuild is a replay with no mapping rules and no upstream reads. The migration drops
the old columns rather than renaming them — EF scaffolded renames that would have carried
ZGW values into columns meaning something else — and empties both tables, since a
pre-slice row is neither reprojectable nor re-derivable from the new source.

Red: HandleAsync recognises a register write but does not yet read or project it, so the
seven projection assertions fail on an empty store.

services/projection-api/Projection.ReadModel/EfNotificationLog.cs

@@ -13,9 +13,8 @@ public sealed class EfNotificationLog(ProjectionDbContext db) : INotificationLog
         db.ProcessedNotifications.Add(new ProcessedNotificationRow
         {
             Key = notification.Key,
-            Actie = notification.Actie,
-            ZaakId = notification.ZaakId,
-            Resource = notification.Resource,
+            RegisterId = notification.RegisterId,
+            Status = notification.Status,
             Reference = notification.Reference,
             ReceivedAt = DateTimeOffset.UtcNow,
         });
@@ -36,6 +35,6 @@ public sealed class EfNotificationLog(ProjectionDbContext db) : INotificationLog
     public async Task<IReadOnlyList<RecordedNotification>> AllAsync(CancellationToken ct = default)
         => await db.ProcessedNotifications
             .OrderBy(r => r.ReceivedAt)
-            .Select(r => new RecordedNotification(r.Key, r.Actie, r.ZaakId, r.Resource, r.Reference))
+            .Select(r => new RecordedNotification(r.Key, r.RegisterId, r.Status, r.Reference))
             .ToListAsync(ct);
 }