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/tracks.sql

@@ -1,10 +1,13 @@
 -- name: UpsertTrack :one
 -- file_path is the canonical identity for library scan; mbid is secondary.
+-- mbid is the file's tag. A looked-up id (mbid_source 'acoustid', M401)
+-- survives a re-read that finds no tag id, and a tag id replaces it (D4).
 INSERT INTO tracks (
     title, album_id, artist_id, track_number, disc_number,
     duration_ms, file_path, file_size, file_format, bitrate, mbid, genre,
-    tag_read_version
-) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13)
+    tag_read_version, mbid_source
+) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13,
+          CASE WHEN $11::text IS NOT NULL THEN 'tag' END)
 ON CONFLICT (file_path) DO UPDATE SET
     title = EXCLUDED.title,
     album_id = EXCLUDED.album_id,
@@ -15,7 +18,11 @@ ON CONFLICT (file_path) DO UPDATE SET
     file_size = EXCLUDED.file_size,
     file_format = EXCLUDED.file_format,
     bitrate = EXCLUDED.bitrate,
-    mbid = EXCLUDED.mbid,
+    -- Both read the row as it was before this statement.
+    mbid = CASE WHEN EXCLUDED.mbid IS NULL AND tracks.mbid_source = 'acoustid'
+                THEN tracks.mbid ELSE EXCLUDED.mbid END,
+    mbid_source = CASE WHEN EXCLUDED.mbid IS NOT NULL THEN 'tag'
+                       WHEN tracks.mbid_source = 'acoustid' THEN 'acoustid' END,
     genre = EXCLUDED.genre,
     -- Stamped on update too, so a tag-repair pass marks rows as done and the
     -- next scan can short-circuit them again (#2499).
@@ -39,7 +46,7 @@ SELECT id, file_path
 -- Heal a track's recording MBID only while still NULL — idempotent, so
 -- re-running the backfill is a no-op for already-healed rows.
 UPDATE tracks
-   SET mbid = $2, updated_at = now()
+   SET mbid = $2, mbid_source = 'tag', updated_at = now()
  WHERE id = $1 AND mbid IS NULL;
 
 -- name: GetTrackByID :one