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.
Register MediaIndex as a harness concern with ALTER TABLE migration to add
service_id column (idempotent). Scope replace_items by service_id (FIXES latent
global-clear bug where building for one Jellyfin wiped another's rows). Scope
query by service_id (empty-string = all rows, backward-compat). Thread
service_id through build_media_index + worker + query_media router. New
regression test proves scoped replace preserves other services' rows.
Backend: 321 pytest pass, ruff clean. Frontend: build green.
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