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/tests/test_services.py

@@ -396,6 +396,77 @@ def test_delete_service_cascades_to_widgets(client, tmp_path):
     assert int(remaining[0]) == 0
 
 
+# ---------------------------------------------------------------------------
+# Harness cascade-delete (Slice 4)
+# ---------------------------------------------------------------------------
+
+
+def test_cascade_delete_removes_harness_data_across_concerns(tmp_path, monkeypatch):
+    """Deleting a service cascades to both qbittorrent samples and media items.
+
+    Proves end-to-end cascade across both harness-managed concerns, and that
+    deleting one service preserves another service's data (multi-instance).
+    """
+    monkeypatch.setenv("BACKEND_CACHE_DIR", str(tmp_path))
+
+    from media_library_viewer_api.services.media_index_impl import MediaIndex
+    from media_library_viewer_api.services.qbittorrent_store import QbittorrentSampleStore
+    from media_library_viewer_api.services.service_data import (
+        get_service_data_harness,
+        reset_service_data_harness,
+    )
+
+    reset_service_data_harness()
+    harness = get_service_data_harness()
+
+    store = SettingsStore(tmp_path / "settings.sqlite")
+    store.ensure_defaults()
+
+    # --- qBit: create service, add samples, delete, verify gone ---
+    qbit_a = store.upsert_service(
+        {"service_type": "qbittorrent", "name": "qA", "config": {"base_url": "http://a"}, "enabled": True}
+    )
+    qbit_b = store.upsert_service(
+        {"service_type": "qbittorrent", "name": "qB", "config": {"base_url": "http://b"}, "enabled": True}
+    )
+    sample_store = QbittorrentSampleStore(harness)
+    sample_store.append(qbit_a["id"], ts=1000, dl_speed=500, up_speed=100)
+    sample_store.append(qbit_b["id"], ts=1000, dl_speed=200, up_speed=50)
+
+    assert len(sample_store.window(qbit_a["id"])) == 1
+    assert len(sample_store.window(qbit_b["id"])) == 1
+
+    store.delete_service(qbit_a["id"])
+
+    assert sample_store.window(qbit_a["id"]) == []
+    assert len(sample_store.window(qbit_b["id"])) == 1  # B survives
+
+    # --- MediaIndex: create services, add items, delete, verify scoped ---
+    jelly_a = store.upsert_service(
+        {"service_type": "jellyfin", "name": "jA", "config": {"base_url": "http://ja"}, "enabled": True}
+    )
+    jelly_b = store.upsert_service(
+        {"service_type": "jellyfin", "name": "jB", "config": {"base_url": "http://jb"}, "enabled": True}
+    )
+    index = MediaIndex(harness.db_path("media_index"))
+    index.init_schema()
+    index.replace_items([{"id": "m1", "title": "A1"}], service_id=jelly_a["id"])
+    index.replace_items([{"id": "m2", "title": "B1"}], service_id=jelly_b["id"])
+
+    rows_a, total_a = index.query(service_id=jelly_a["id"])
+    rows_b, total_b = index.query(service_id=jelly_b["id"])
+    assert total_a == 1 and total_b == 1
+
+    store.delete_service(jelly_a["id"])
+
+    rows_a_after, total_a_after = index.query(service_id=jelly_a["id"])
+    rows_b_after, total_b_after = index.query(service_id=jelly_b["id"])
+    assert total_a_after == 0  # deleted
+    assert total_b_after == 1  # survives
+
+    reset_service_data_harness()
+
+
 # ---------------------------------------------------------------------------
 # Service task run history
 # ---------------------------------------------------------------------------