fix: use ADTS for AAC transcoding, temporarily exclude AAC from transcode decisions (#5167)

* fix: use ADTS format for AAC transcoding to avoid silent output on ffmpeg 8.0+

The fragmented MP4 muxer (`-f ipod -movflags frag_keyframe+empty_moov`)
produces corrupt/silent audio when ffmpeg pipes to stdout, confirmed on
ffmpeg 8.0+. The moof atom offset values are zeroed out in pipe mode,
causing AAC decoder errors. Switch to `-f adts` (raw AAC framing) which
works reliably via pipe and is widely supported by clients including
UPnP/Sonos devices.

* fix: exclude AAC from transcode decision, as it is not working for Sonos.

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

db/migrations/20260310113858_fix_aac_transcode_command.go

@@ -0,0 +1,30 @@
+package migrations
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/pressly/goose/v3"
+)
+
+func init() {
+	goose.AddMigrationContext(upFixAacTranscodeCommand, downFixAacTranscodeCommand)
+}
+
+func upFixAacTranscodeCommand(_ context.Context, tx *sql.Tx) error {
+	// The old AAC command used `-f ipod -movflags frag_keyframe+empty_moov` which produces
+	// corrupt/silent audio when ffmpeg pipes to stdout (confirmed in ffmpeg 8.0+).
+	// Switch to `-f adts` (raw AAC framing) which works reliably via pipe.
+	// Only update rows that still have the old default command.
+	const oldCommand = "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f ipod -movflags frag_keyframe+empty_moov -"
+	const newCommand = "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -"
+	_, err := tx.Exec(
+		"UPDATE transcoding SET command = ? WHERE target_format = 'aac' AND command = ?",
+		newCommand, oldCommand,
+	)
+	return err
+}
+
+func downFixAacTranscodeCommand(_ context.Context, tx *sql.Tx) error {
+	return nil
+}