feat(service-storage-harness): slice 4 — cascade-delete wiring + integration test

Wire ServiceDataHarness.cascade_delete into SettingsStore.delete_service
(best-effort try/except, logs on failure). Fix migration runner to also
catch 'no such table' on fresh DBs (ALTER TABLE before init_schema).
Integration test proves end-to-end cascade across both concerns (qBit
samples + media items) with multi-instance preservation.

Backend: 322 pytest pass, ruff clean.

backend/src/media_library_viewer_api/services/service_data.py

@@ -95,8 +95,9 @@ class ServiceDataHarness:
                         try:
                             conn.execute(stmt)
                         except sqlite3.OperationalError as exc:
-                            if "duplicate column name" in str(exc).lower():
-                                logger.debug("Skipping already-applied migration: %s", stmt[:80])
+                            lowered = str(exc).lower()
+                            if "duplicate column name" in lowered or "no such table" in lowered:
+                                logger.debug("Skipping migration (already applied or table absent): %s", stmt[:80])
                             else:
                                 raise
                 conn.commit()