feat(library): AcoustID lookup worker fills the MBIDs tags leave empty (M401 #3920 #3921)

Migration 0069 adds tracks.mbid_source (tag | acoustid), a lookup state per
track (matched | ambiguous | no_match | failed) and the acoustid_settings
row (off, no key, min score 0.85).

The file's tag outranks a lookup (D4). UpsertTrack keeps a looked-up id
through a re-read that finds no tag id and replaces it as soon as one
appears. SetTrackMbidFromAcoustID refuses to write over a tag id.

The worker fingerprints each untagged track with fpcalc's compressed
print, looks it up and writes an id only when D5 settles it: one
recording at or above the threshold, or one left after matching title and
length. Ambiguous and no-match results write nothing. A key AcoustID
refuses, or the service being unreachable, stops the pass and is reported
in the worker's status. It never counts as a verdict on a track.

A changed file drops its lookup in the scan. The re-lookup takes back an
id that no longer matches.

Admin API: GET /api/admin/library/acoustid (settings, status, coverage by
source), PUT …/acoustid-settings (write-only key), POST …/acoustid/run,
GET …/acoustid/unsettled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/queries/merge.sql

@@ -88,7 +88,7 @@ ON CONFLICT (track_a_id, track_b_id, source) DO NOTHING;
 -- A recording MBID is what the similarity pipeline keys on. If only the removed
 -- copy carried one, the survivor takes it rather than going dark to similarity.
 UPDATE tracks AS survivor
-   SET mbid = loser.mbid
+   SET mbid = loser.mbid, mbid_source = loser.mbid_source
   FROM tracks AS loser
  WHERE survivor.id = sqlc.arg(survivor_id)::uuid
    AND loser.id = sqlc.arg(loser_id)::uuid