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/dbq/events.sql.go

@@ -261,7 +261,7 @@ func (q *Queries) InsertSkipEvent(ctx context.Context, arg InsertSkipEventParams
 }
 
 const listRecentSessionTracks = `-- name: ListRecentSessionTracks :many
-SELECT t.id, t.title, t.album_id, t.artist_id, t.track_number, t.disc_number, t.duration_ms, t.file_path, t.file_size, t.file_format, t.bitrate, t.mbid, t.genre, t.added_at, t.updated_at, t.tag_source, t.tag_sources_version, t.tag_read_version, t.missing_since FROM tracks t
+SELECT t.id, t.title, t.album_id, t.artist_id, t.track_number, t.disc_number, t.duration_ms, t.file_path, t.file_size, t.file_format, t.bitrate, t.mbid, t.genre, t.added_at, t.updated_at, t.tag_source, t.tag_sources_version, t.tag_read_version, t.missing_since, t.mbid_source FROM tracks t
 JOIN play_events pe ON pe.track_id = t.id
 WHERE pe.session_id = $1
   AND pe.started_at < $2
@@ -307,6 +307,7 @@ func (q *Queries) ListRecentSessionTracks(ctx context.Context, arg ListRecentSes
 			&i.TagSourcesVersion,
 			&i.TagReadVersion,
 			&i.MissingSince,
+			&i.MbidSource,
 		); err != nil {
 			return nil, err
 		}