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/event-subscriber/EventSubscriber.Application/Notification.cs

@@ -2,11 +2,16 @@ namespace EventSubscriber.Application;
 
 /// <summary>
 /// An inbound NRC (Open Notificaties) notification, as Open Notificaties POSTs it to an
-/// abonnement callback. Only the fields the projection needs are modelled; the full ZGW
-/// "Notificatie" resource also carries <c>aanmaakdatum</c> and <c>kenmerken</c> which the
-/// minimal projection ignores (bsn is deferred — see ADR-0008). For a <c>zaken</c>/<c>zaak</c>/<c>create</c>
-/// notification <c>hoofdObject</c> and <c>resourceUrl</c> are both the created zaak's URL.
+/// abonnement callback. Only the fields the projection needs are modelled.
 /// </summary>
+/// <remarks>
+/// Since S-19b-2 the subscriber listens on the <c>objecten</c> kanaal, not <c>zaken</c>: the
+/// register record in Objecten is what the projection is derived from (ADR-0030), so the
+/// projection is a cache of the register rather than a re-derivation of the case system. An
+/// Objecten notification carries <b>no record data</b> — only the object URL (as both
+/// <c>hoofdObject</c> and <c>resourceUrl</c>) and the objecttype as a kenmerk — so the record
+/// itself is read back through the ACL.
+/// </remarks>
 public sealed record Notification(
     string Kanaal,
     string Resource,
@@ -14,28 +19,12 @@ public sealed record Notification(
     Uri ResourceUrl,
     Uri? HoofdObject = null)
 {
-    /// <summary>A zaak being created — projected as INGEDIEND.</summary>
-    public bool IsZaakCreated =>
-        Kanaal == "zaken" && Resource == "zaak" && Actie == "create";
+    /// <summary>A register record written to Objecten — <c>create</c> on submit, <c>update</c> on
+    /// approval, since the ACL upserts the same object for a registration (§8.6).</summary>
+    public bool IsRegisterRecordWritten =>
+        Kanaal == "objecten" && Resource == "object" && Actie is "create" or "update";
 
-    /// <summary>A status being set on a zaak — the approval, projected as INGESCHREVEN (S-09b). In the
-    /// walking skeleton the only status ever set after creation is the approval, and the subscriber may
-    /// not read OpenZaak (§8.1), so any status-create is taken as the approval.</summary>
-    public bool IsZaakStatusSet =>
-        Kanaal == "zaken" && Resource == "status" && Actie == "create";
-
-    /// <summary>The zaak URL this notification concerns — <c>hoofdObject</c> (the zaak) for a status
-    /// notification, else the resource URL (which, for a zaak-create, is the zaak).</summary>
-    public Uri ZaakUrl => HoofdObject ?? ResourceUrl;
-
-    /// <summary>The zaak UUID used as the projection key — the trailing segment of <see cref="ZaakUrl"/>.</summary>
-    public string ZaakId => ZaakUrl.Segments[^1].Trim('/');
-
-    /// <summary>
-    /// A deterministic dedup key. Open Notificaties carries no notification id and may
-    /// redeliver, so the key is derived from the immutable notification content: two
-    /// deliveries of the same zaak-create collapse to one. (NRC may also deliver
-    /// out of order; the projector tolerates that — order does not change the outcome.)
-    /// </summary>
-    public string IdempotencyKey => $"{Kanaal}:{Resource}:{Actie}:{ResourceUrl}";
+    /// <summary>The object holding the register record. Objecten sets both fields to the object;
+    /// <c>hoofdObject</c> is the main resource by definition, so prefer it.</summary>
+    public Uri ObjectUrl => HoofdObject ?? ResourceUrl;
 }