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()

backend/src/media_library_viewer_api/services/settings_store.py

@@ -1740,7 +1740,7 @@ class SettingsStore:
         return self.get_service(service["id"]) or service
 
     def delete_service(self, service_id: str) -> None:
-        """Delete a service and cascade-delete widgets referencing it."""
+        """Delete a service and cascade-delete widgets + harness data."""
         self.init_schema()
         with self.connect() as conn:
             # The service_id column on dashboard_widgets is added in a later
@@ -1753,6 +1753,15 @@ class SettingsStore:
                 )
             conn.execute("DELETE FROM services WHERE id = ?", (service_id,))
 
+        # Cascade-delete harness-managed data (best-effort: the service row is
+        # already removed; data cleanup must not block service deletion).
+        try:
+            from media_library_viewer_api.services.service_data import get_service_data_harness
+
+            get_service_data_harness().cascade_delete(service_id)
+        except Exception:
+            logger.exception("Failed to cascade-delete harness data for service %s", service_id)
+
     def record_service_task_run(self, payload: dict[str, Any]) -> dict[str, Any]:
         """Append a service task run history row."""
         self.init_schema()