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/db/dbq/models.go

@@ -417,6 +417,13 @@ type LidarrRequest struct {
 	LidarrAddConfirmedAt pgtype.Timestamptz
 }
 
+type LoudnessSetting struct {
+	ID                  bool
+	Enabled             bool
+	BackfillConcurrency int32
+	UpdatedAt           pgtype.Timestamptz
+}
+
 type MissingReacquisition struct {
 	AlbumID       pgtype.UUID
 	Attempts      int32
@@ -713,6 +720,18 @@ type TrackFingerprint struct {
 	ChromaprintLengthSec int32
 }
 
+type TrackLoudness struct {
+	TrackID         pgtype.UUID
+	IntegratedLufs  *float32
+	TruePeakDbtp    *float32
+	LoudnessRangeLu *float32
+	BlockHistStart  *int16
+	BlockHist       []int32
+	Unreadable      bool
+	AnalysisVersion int16
+	AnalyzedAt      pgtype.Timestamptz
+}
+
 type TrackSimilarity struct {
 	TrackAID  pgtype.UUID
 	TrackBID  pgtype.UUID