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>

web/src/lib/api/admin.ts

@@ -364,6 +364,38 @@ export async function updateFingerprintSettings(
   return api.put<FingerprintSettings>('/api/admin/library/fingerprint-settings', s);
 }
 
+// Loudness analysis (#4995) -------------------------------------------------
+
+export type LoudnessCoverage = {
+  total: number;
+  measured: number;
+  // Read fine, but no part of the track was loud enough to measure: silence,
+  // or a file shorter than half a second.
+  silent: number;
+  unreadable: number;
+  pending: number;
+  // False when the operator has switched analysis off: pending then never
+  // shrinks, and nothing should read as progress.
+  enabled: boolean;
+};
+
+export async function getLoudnessCoverage(): Promise<LoudnessCoverage> {
+  return api.get<LoudnessCoverage>('/api/admin/library/loudness');
+}
+
+export type LoudnessSettings = {
+  enabled: boolean;
+  backfill_concurrency: number;
+};
+
+export async function getLoudnessSettings(): Promise<LoudnessSettings> {
+  return api.get<LoudnessSettings>('/api/admin/library/loudness-settings');
+}
+
+export async function updateLoudnessSettings(s: LoudnessSettings): Promise<LoudnessSettings> {
+  return api.put<LoudnessSettings>('/api/admin/library/loudness-settings', s);
+}
+
 // Cover-art providers ------------------------------------------------------
 
 export type CoverProviderCapability = 'album_cover' | 'artist_thumb' | 'artist_fanart';