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/api/api.go

@@ -35,7 +35,7 @@ import (
 // Mount attaches /api/* handlers to r. Public endpoints (login) are outside
 // RequireUser; everything else is gated by the middleware. The events writer
 // is shared with the Subsonic mount so /rest/scrobble feeds the same store.
-func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, recSettings *recsettings.Service, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, tagSettings *tags.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte, netSettings *netsettings.Service, reacqSettings *reacquisition.SettingsService, fpSettings *library.FingerprintSettingsService, loudSettings *library.LoudnessSettingsService) {
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, recSettings *recsettings.Service, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, tagSettings *tags.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte, netSettings *netsettings.Service, reacqSettings *reacquisition.SettingsService, fpSettings *library.FingerprintSettingsService, loudSettings *library.LoudnessSettingsService, acoustIDLookup *library.AcoustIDLookupWorker) {
 	rng := rand.New(rand.NewSource(rand.Int63()))
 	setupToken, err := auth.NewSetupToken()
 	if err != nil {
@@ -67,6 +67,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		reacqSettings:       reacqSettings,
 		fingerprintSettings: fpSettings,
 		loudnessSettings:    loudSettings,
+		acoustIDLookup:      acoustIDLookup,
 		leveled:             newLeveledRenderer(dataDir, loudSettings, logger),
 		librarySize:         recommendation.NewLibrarySize(nil),
 		loginGuard:          auth.NewLoginGuard(),
@@ -251,6 +252,10 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Get("/library/loudness", h.handleGetLoudnessCoverage)
 				admin.Get("/library/loudness-settings", h.handleGetLoudnessSettings)
 				admin.Put("/library/loudness-settings", h.handleUpdateLoudnessSettings)
+				admin.Get("/library/acoustid", h.handleGetAcoustID)
+				admin.Put("/library/acoustid-settings", h.handleUpdateAcoustIDSettings)
+				admin.Post("/library/acoustid/run", h.handleRunAcoustIDLookup)
+				admin.Get("/library/acoustid/unsettled", h.handleListUnsettledAcoustID)
 				// Duplicates report (#3912): proposals from the duplicate sweep, a
 				// trigger to sweep now, dismissal, and the merge (#3911), which deletes
 				// the removed copies' files after moving their history onto the kept one.
@@ -348,6 +353,9 @@ type handlers struct {
 	// loudnessSettings is the loudness analysis policy (M464 #4995), the same
 	// instance the loudness backfill reads. Nil serves the defaults.
 	loudnessSettings *library.LoudnessSettingsService
+	// acoustIDLookup is the AcoustID lookup worker (M401): its settings, its
+	// status and "look up now". Nil serves the defaults (off).
+	acoustIDLookup *library.AcoustIDLookupWorker
 	// leveled renders the leveled streams handed to Sonos/UPnP speakers
 	// (M464 #5001). Nil when its cache directory cannot be made: a level
 	// request then gets the plain stream.