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>

cmd/minstrel/main.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/jackc/pgx/v5/pgxpool"
 
+	"git.fabledsword.com/bvandeusen/minstrel/internal/acoustid"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/config"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coplay"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
@@ -139,6 +140,15 @@ func run() error {
 		logger.Warn("loudness settings: using defaults", "err", loudErr)
 	}
 
+	// AcoustID lookup settings (M401): shared by the lookup worker and the
+	// admin API. A load failure leaves the lookup off.
+	acoustIDSettings, acoustIDErr := library.NewAcoustIDSettingsService(ctx, pool)
+	if acoustIDErr != nil {
+		logger.Warn("acoustid settings: using defaults (lookup off)", "err", acoustIDErr)
+	}
+	acoustIDLookup := library.NewAcoustIDLookupWorker(pool, logger.With("component", "acoustid_lookup"),
+		acoustIDSettings, acoustid.New(acoustid.DefaultBaseURL))
+
 	contact := cfg.Library.ContactEmail
 	if contact == "" {
 		contact = "https://git.fabledsword.com/bvandeusen/minstrel"
@@ -240,6 +250,11 @@ func run() error {
 	// measurement. See internal/library/loudness_backfill.go.
 	go library.NewLoudnessBackfillWorker(pool, logger.With("component", "loudness_backfill"), loudSettings).Run(ctx)
 
+	// AcoustID lookup (M401): fills the recording MBIDs untagged tracks lack,
+	// so the similarity arms can see them. Idle until the operator adds a key
+	// and switches it on. See internal/library/acoustid_lookup.go.
+	go acoustIDLookup.Run(ctx)
+
 	// Duplicate sweep (M400 #3910): proposes groups of tracks holding one
 	// recording, from the fingerprints above. Sweeps only when fingerprints have
 	// changed since the last sweep.
@@ -390,6 +405,7 @@ func run() error {
 	srv.TagSettings = tagSettings
 	srv.FingerprintSettings = fpSettings
 	srv.LoudnessSettings = loudSettings
+	srv.AcoustIDLookup = acoustIDLookup
 	// The sweeper above holds this same instance, so a save from the admin
 	// card changes what it does on its next tick (#3936).
 	srv.ReacqSettings = reacqSettings