New core/replaygain.py module uses FFmpeg's ebur128 filter (already a
project dependency) to analyze integrated loudness and true peak, then
writes ReplayGain 2.0 tags (-18 LUFS reference) to MP3 (TXXX frames),
FLAC/OGG/Opus (Vorbis comments), and M4A/MP4 (freeform atoms).
Three analysis modes in the enhanced library view:
- Per-track RG button: synchronous single-track analysis (~1-3 s)
- Album "ReplayGain" button: background job writing both track gain
and album gain (mean LUFS across all album tracks) to every file
- Bulk bar "ReplayGain" button: batch track-gain for selected tracks
read_file_tags() in tag_writer.py extended with four new optional keys
(replaygain_track_gain/_peak, replaygain_album_gain/_peak) so existing
RG values surface in the tag-preview diff view. Purely additive — no
existing endpoints or DB schema changed.
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