Post-processing applies ReplayGain only to slskd/WebUI downloads — content added
via Lidarr, the REST API, or by hand never got it, and there was no way to (re)apply
RG to existing tracks or fix ones where analysis failed (raised in #437 + comments).
New ReplayGain Filler repair job (sibling of Lyrics/Cover Art fillers): scans for
tracks with no ReplayGain track-gain tag and creates a finding per track; the scan
only READS tags (cheap) and no-ops when ffmpeg is absent. Applying a finding runs
the same ffmpeg ebur128 analysis the import pipeline uses (gain = ref - LUFS) and
writes the RG tags in place — no moves, no re-matching. Opt-in (default off),
schedulable like the other maintenance jobs.
Wired: job registry, repair_worker apply handler (_fix_missing_replaygain) +
fixable-types, and the findings UI (label / fix-button / detail rows).
Tests: pure needs_replaygain decision (missing/blank/present/+0.00-is-tagged) +
the apply handler's analyze→compute→write seam with the pipeline gain formula,
ffmpeg-absent + missing-file guards, and registration. 93 repair tests green.
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