feat(library): measure every track's loudness in the background (M464 #4995)

The first step of loudness normalization: the server measures each track
with ffmpeg's EBU R128 filter (true peak, mono as dual mono) and stores the
integrated loudness, true peak and loudness range in track_loudness
(migration 0065).

It also keeps a histogram of the 400 ms gating blocks at 0.1 LU, so album
loudness can be computed exactly later with no second decode (#4996). The
histogram reproduces ffmpeg's own figure (-10.68 against -10.7 on the
captured fixture), and the analyzer logs a warning if the two ever drift.

- A background worker, cloned from the fingerprint backfill, measures every
  track, new ones included. Measuring inline in the scan was dropped: the
  analysis decodes the whole file, and a large import could pass the scan's
  one-hour stuck threshold. The scan only deletes a changed file's
  measurement; the worker ticks every 10 minutes.
- Timeouts, the cancel/missing-binary split and settled verdicts follow the
  fingerprint runner. Silence and undecodable files are stored as verdicts;
  stalls are retried. The deadline scales with track length.
- loudness_settings (enabled, files at once) and an admin card with the
  coverage gauge, under GET/PUT /api/admin/library/loudness-settings and
  GET /api/admin/library/loudness.

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

internal/api/api.go

@@ -34,7 +34,7 @@ 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) {
+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) {
 	rng := rand.New(rand.NewSource(rand.Int63()))
 	setupToken, err := auth.NewSetupToken()
 	if err != nil {
@@ -65,6 +65,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		netSettings:         netSettings,
 		reacqSettings:       reacqSettings,
 		fingerprintSettings: fpSettings,
+		loudnessSettings:    loudSettings,
 		librarySize:         recommendation.NewLibrarySize(nil),
 		loginGuard:          auth.NewLoginGuard(),
 		setupToken:          setupToken,
@@ -237,6 +238,9 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Get("/library/fingerprints", h.handleGetFingerprintCoverage)
 				admin.Get("/library/fingerprint-settings", h.handleGetFingerprintSettings)
 				admin.Put("/library/fingerprint-settings", h.handleUpdateFingerprintSettings)
+				admin.Get("/library/loudness", h.handleGetLoudnessCoverage)
+				admin.Get("/library/loudness-settings", h.handleGetLoudnessSettings)
+				admin.Put("/library/loudness-settings", h.handleUpdateLoudnessSettings)
 				// 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.
@@ -331,6 +335,9 @@ type handlers struct {
 	// instance the scanner and the fingerprint workers read, so a save from the
 	// admin card reaches them without a restart. Nil serves the defaults.
 	fingerprintSettings *library.FingerprintSettingsService
+	// loudnessSettings is the loudness analysis policy (M464 #4995), the same
+	// instance the loudness backfill reads. Nil serves the defaults.
+	loudnessSettings *library.LoudnessSettingsService
 	// setupToken must accompany the first registration while no users exist
 	// (see auth.SetupToken). requireSetupToken is set by Mount, the only
 	// production constructor; tests that build handlers directly leave it