Adds the third resolver stage for tracks the discovery cache + library can't resolve — a live
search of the target service, gated behind a "Match missing tracks" toggle so the API cost is opt-in.
The whole point is coverage WITHOUT the wrong-track risk, so it's a CONFIDENT match, not "search
and grab":
- search_service_track_id(artist, title, search_fn): searches the service, reranks via the existing
relevance scorer (filter_and_rerank), and returns the top hit's id ONLY if it clears
BACKFILL_MIN_SCORE (1.2 on the score_track scale). A wrong-artist hit (no 1.5x exact-artist boost,
caps ~1.0) or a karaoke/cover (x0.05) can't clear the floor → None, and the track is left out
rather than added wrong. search_fn injected → unit-testable without a live service.
- resolve_service_track_ids gains an optional search_id_fn: cache → library → search. Tallies
from_search separately.
- _run_service_export builds the search fn from the service's metadata search client only when
job['backfill'] is set; the endpoint reads `backfill` from the body; the modal adds the toggle and
the status line shows "(N matched live)".
Store-back of confident matches deferred: a mirrored-only track may have no library row to write to,
so persisting needs the track→library mapping — a follow-up, not correctness.
9 new tests incl. the safety ones: wrong-artist rejected, karaoke/cover rejected, real-over-cover
picked, fail-safe on search error, and the cache→library→search waterfall + toggle wiring (on/off).
28 export/orchestration tests green, 64 script-integrity green, ruff clean.
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