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/library/scanner.go

@@ -402,6 +402,13 @@ func (s *Scanner) scanFile(
 			// must not outlive them, or the sweep would compare audio that is gone.
 			s.logger.Warn("fingerprint: clearing stale fingerprint failed", "path", path, "err", err)
 		}
+		// Loudness is measured by its own worker, never inline: it decodes the
+		// whole file (see loudness_backfill.go). The scan's part is to drop a
+		// measurement of bytes that are gone, so clients stop leveling this
+		// track by the old file's loudness and the worker measures it again.
+		if err := q.DeleteTrackLoudness(ctx, track.ID); err != nil {
+			s.logger.Warn("loudness: clearing stale measurement failed", "path", path, "err", err)
+		}
 	}
 
 	if knownTrack {