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.
Developer
75c949ad25