feat: leveled FLAC stream for Sonos/UPnP speakers (M464 #5001)

Speakers fetch their own audio, so the phone cannot level it. A cast
token minted with level=true (and the client's asAlbum, which only the
queue holder knows) now returns GET /api/tracks/{id}/leveled.flac: the
track rendered by ffmpeg at the user's gain (volume=XdB, plus
alimiter at -1 dBFS for a limiter-mode boost), metadata stripped, FLAC
at 16 or 24 bits and at most 48 kHz. The gain is computed server-side
from the user's preference and the stored loudness, carried as
?g=<centi-dB>&lim=0|1 and signed into the token, so an edited URL does
not verify. Unity gains get the plain stream.

Renders are written beside the cache file and renamed in, keyed by the
source's size and mtime, coalesced per file (singleflight, detached
from the requesting speaker so a retry finds the render running),
started at mint time so the fetch finds them ready, and evicted least
recently used past leveled_cache_mb, a new admin setting (migration
0068, Loudness analysis card).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/loudness_settings.go

@@ -22,17 +22,27 @@ type LoudnessSettings struct {
 	// values, so normalization keeps working for them.
 	Enabled             bool
 	BackfillConcurrency int32
+	// LeveledCacheMB caps the disk the leveled-stream renders for Sonos and
+	// UPnP speakers may use (#5001); the least recently played go first.
+	LeveledCacheMB int32
 	// UpdatedAt is set by the database; ignored by Set.
 	UpdatedAt time.Time
 }
 
-// DefaultLoudnessSettings mirrors migration 0065's column defaults, so a
+// DefaultLoudnessSettings mirrors migrations 0065 and 0068's column defaults, so a
 // database that cannot be read still analyzes the way a fresh install does.
 var DefaultLoudnessSettings = LoudnessSettings{
 	Enabled:             true,
 	BackfillConcurrency: loudnessBackfillConcurrency,
+	LeveledCacheMB:      2048,
 }
 
+// The leveled-stream cache's bounds, as migration 0068's CHECK has them.
+const (
+	minLeveledCacheMB = 256
+	maxLeveledCacheMB = 65536
+)
+
 // ErrLoudnessSettingOutOfRange is returned by Set for a value migration 0065's
 // CHECK would reject, so the API answers 400 naming the field.
 var ErrLoudnessSettingOutOfRange = errors.New("loudness setting out of range")
@@ -78,6 +88,7 @@ func (s *LoudnessSettingsService) Set(ctx context.Context, in LoudnessSettings)
 	row, err := dbq.New(s.pool).UpdateLoudnessSettings(ctx, dbq.UpdateLoudnessSettingsParams{
 		Enabled:             in.Enabled,
 		BackfillConcurrency: in.BackfillConcurrency,
+		LeveledCacheMb:      in.LeveledCacheMB,
 	})
 	if err != nil {
 		return LoudnessSettings{}, fmt.Errorf("loudness settings: save: %w", err)
@@ -94,6 +105,10 @@ func validateLoudnessSettings(in LoudnessSettings) error {
 		return fmt.Errorf("%w: backfill_concurrency must be %d-%d",
 			ErrLoudnessSettingOutOfRange, minBackfillConcurrency, maxBackfillConcurrency)
 	}
+	if in.LeveledCacheMB < minLeveledCacheMB || in.LeveledCacheMB > maxLeveledCacheMB {
+		return fmt.Errorf("%w: leveled_cache_mb must be %d-%d",
+			ErrLoudnessSettingOutOfRange, minLeveledCacheMB, maxLeveledCacheMB)
+	}
 	return nil
 }
 
@@ -101,6 +116,7 @@ func loudnessSettingsFromRow(row dbq.LoudnessSetting) LoudnessSettings {
 	return LoudnessSettings{
 		Enabled:             row.Enabled,
 		BackfillConcurrency: row.BackfillConcurrency,
+		LeveledCacheMB:      row.LeveledCacheMb,
 		UpdatedAt:           row.UpdatedAt.Time,
 	}
 }