v2.53.1 — MED-3: Tests für Tombstone-Heuristik, SyncBericht, songAusServerMap + delete-deleted_at

## MED-3 (Review-Fix)
- test/sync_service_test.dart NEU (14 Tests): tombstoneAnwenden-Matrix
  (letzterSync null → true, deletedAt null → true, älter → false, neuer →
  true, Gleichstand → false, ungültiger Timestamp → true), SyncBericht
  (hatAenderungen, zusammenfassung kombiniert/leer, details), songAusServerMap
  (cloud_id/Titel/Künstler/Dauer/Größe/istKorrupt-Mapping, Defaults bei
  fehlenden Feldern, String-Robustheit) — deckt die MED-1-Heuristik + den
  MED-2-DB-Insert ab (123→140 Tests)
- test/cloud_service_test.dart +3: delete mit deletedAt sendet deleted_at im
  Body, delete ohne deletedAt sendet KEIN deleted_at-Feld, delete
  Netzwerkfehler → false

test/cloud_service_test.dart

@@ -304,6 +304,44 @@ void main() {
       expect(await cloud401.delete('s1'), isFalse);
     });
 
+    test('delete mit deletedAt sendet deleted_at im Body (Tombstone, Sprint E)',
+        () async {
+      late Map<String, dynamic> gesendeterBody;
+      final cloud = CloudService(
+        client: MockClient((req) async {
+          gesendeterBody = jsonDecode(req.body) as Map<String, dynamic>;
+          return http.Response('{}', 200);
+        }),
+      );
+      final ok = await cloud.delete('s1', deletedAt: '2026-08-05T10:00:00');
+      expect(ok, isTrue);
+      expect(gesendeterBody['song_id'], 's1');
+      expect(gesendeterBody['deleted_at'], '2026-08-05T10:00:00');
+    });
+
+    test('delete ohne deletedAt sendet KEIN deleted_at-Feld', () async {
+      late Map<String, dynamic> gesendeterBody;
+      final cloud = CloudService(
+        client: MockClient((req) async {
+          gesendeterBody = jsonDecode(req.body) as Map<String, dynamic>;
+          return http.Response('{}', 200);
+        }),
+      );
+      final ok = await cloud.delete('s1');
+      expect(ok, isTrue);
+      expect(gesendeterBody['song_id'], 's1');
+      expect(gesendeterBody.containsKey('deleted_at'), isFalse);
+    });
+
+    test('delete Netzwerkfehler → false (kein Crash)', () async {
+      final cloud = CloudService(
+        client: MockClient(
+            (req) async => throw http.ClientException('offline')),
+      );
+      expect(await cloud.delete('s1', deletedAt: '2026-08-05T10:00:00'),
+          isFalse);
+    });
+
     test('globalList 200 → Songs, 401 → []', () async {
       final cloudOk = CloudService(
         client: MockClient((req) async =>