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/dbtest/reset.go

@@ -91,6 +91,7 @@ var dataTables = []string{
 	"duplicate_groups",
 	"duplicate_sweeps",
 	"track_fingerprints", // M400
+	"track_loudness",     // M464
 	"tracks",
 	"albums",
 	"artists",
@@ -141,4 +142,11 @@ func ResetDB(t *testing.T, pool *pgxpool.Pool) {
 	); err != nil {
 		t.Fatalf("dbtest.ResetDB reset fingerprint settings: %v", err)
 	}
+	// Loudness analysis settings (M464 #4995), reset the same way.
+	if _, err := pool.Exec(ctx, `
+		UPDATE loudness_settings
+		   SET enabled = DEFAULT, backfill_concurrency = DEFAULT, updated_at = DEFAULT`,
+	); err != nil {
+		t.Fatalf("dbtest.ResetDB reset loudness settings: %v", err)
+	}
 }