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/Migrations/ProjectionDbContextModelSnapshot.cs

@@ -28,11 +28,6 @@ namespace Projection.ReadModel.Migrations
                         .HasColumnType("text")
                         .HasColumnName("key");
 
-                    b.Property<string>("Actie")
-                        .IsRequired()
-                        .HasColumnType("text")
-                        .HasColumnName("actie");
-
                     b.Property<DateTimeOffset>("ReceivedAt")
                         .HasColumnType("timestamp with time zone")
                         .HasColumnName("received_at");
@@ -41,15 +36,15 @@ namespace Projection.ReadModel.Migrations
                         .HasColumnType("text")
                         .HasColumnName("reference");
 
-                    b.Property<string>("Resource")
+                    b.Property<string>("RegisterId")
                         .IsRequired()
                         .HasColumnType("text")
-                        .HasColumnName("resource");
+                        .HasColumnName("register_id");
 
-                    b.Property<string>("ZaakId")
+                    b.Property<string>("Status")
                         .IsRequired()
                         .HasColumnType("text")
-                        .HasColumnName("zaak_id");
+                        .HasColumnName("status");
 
                     b.HasKey("Key");