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>

cmd/minstrel/main.go

@@ -132,6 +132,13 @@ func run() error {
 	}
 	scanner := library.New(pool, logger, cfg.Library.ScanPaths, fpSettings)
 
+	// Loudness analysis settings (M464 #4995): shared by the loudness backfill
+	// and the admin API, and falls back to the defaults like the above.
+	loudSettings, loudErr := library.NewLoudnessSettingsService(ctx, pool)
+	if loudErr != nil {
+		logger.Warn("loudness settings: using defaults", "err", loudErr)
+	}
+
 	contact := cfg.Library.ContactEmail
 	if contact == "" {
 		contact = "https://git.fabledsword.com/bvandeusen/minstrel"
@@ -228,6 +235,11 @@ func run() error {
 	// internal/library/fingerprint_backfill.go for why.
 	go library.NewFingerprintBackfillWorker(pool, logger.With("component", "fingerprint_backfill"), fpSettings).Run(ctx)
 
+	// Loudness backfill (M464 #4995): measures every track's loudness for
+	// normalization, new tracks included; the scan only drops a changed file's
+	// measurement. See internal/library/loudness_backfill.go.
+	go library.NewLoudnessBackfillWorker(pool, logger.With("component", "loudness_backfill"), loudSettings).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.
@@ -377,6 +389,7 @@ func run() error {
 	srv.RecSettings = recSettings
 	srv.TagSettings = tagSettings
 	srv.FingerprintSettings = fpSettings
+	srv.LoudnessSettings = loudSettings
 	// 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