test(audiobooks): tighten delete tests with cascade + unreachability checks

Final review flagged two small gaps in the delete tests:
- TestCollection_Delete_Owner_204 now seeds a book and asserts the
  items table is empty after delete (proves FK CASCADE works).
- TestPlaylist_Delete_Owner_FiresRemovedEvent now also asserts the
  post-delete GET returns 404 (symmetry with the collection test).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/audiobooks/abs/collections_handler_test.go

@@ -433,9 +433,13 @@ func TestCollection_Patch_NonOwner_404(t *testing.T) {
 }
 
 func TestCollection_Delete_Owner_204(t *testing.T) {
-	hb := newCollectionsHarness(t)
+	hb := newCollectionsHarness(t, "book-1")
 	id := createCollectionForUser(t, hb, "1", "", `{"name":"x"}`)
 
+	// Seed an item so the cascade-delete is exercised.
+	_ = dispatchABSWithParams(http.MethodPost, "/api/collections/"+id+"/book/book-1",
+		map[string]string{"id": id, "bookId": "book-1"}, nil, "1", "", hb.H.handleAddCollectionBook)
+
 	rec := dispatchABSWithParams(http.MethodDelete, "/api/collections/"+id, map[string]string{"id": id}, nil, "1", "", hb.H.handleDeleteCollection)
 	if rec.Code != http.StatusNoContent {
 		t.Fatalf("status = %d, want 204; body=%s", rec.Code, rec.Body.String())
@@ -445,6 +449,11 @@ func TestCollection_Delete_Owner_204(t *testing.T) {
 	if rec2.Code != http.StatusNotFound {
 		t.Errorf("post-delete GET status = %d, want 404", rec2.Code)
 	}
+	// Cascade: items table must be empty for the deleted collection.
+	items, _ := hb.Coll.ListCollectionItems(context.Background(), id)
+	if len(items) != 0 {
+		t.Errorf("items len = %d, want 0 (cascade did not drop child rows)", len(items))
+	}
 }
 
 func TestCollection_Delete_NonOwner_404(t *testing.T) {

internal/audiobooks/abs/playlists_handler_test.go

@@ -370,6 +370,11 @@ func TestPlaylist_Delete_Owner_FiresRemovedEvent(t *testing.T) {
 	if evts[len(evts)-1].Event != "playlist_removed" {
 		t.Errorf("event = %q, want playlist_removed", evts[len(evts)-1].Event)
 	}
+	// Post-delete GET must 404 (symmetry with collection delete test).
+	rec2 := dispatchABSWithParams(http.MethodGet, "/api/playlists/"+id, map[string]string{"id": id}, nil, "7", "", hb.H.handleGetPlaylist)
+	if rec2.Code != http.StatusNotFound {
+		t.Errorf("post-delete GET status = %d, want 404", rec2.Code)
+	}
 }
 
 func TestPlaylist_Delete_NonOwner_404(t *testing.T) {