Fix OpenSubsonic ReplayGain loudness calculation (#2893)

Fix OpenSubsonic ReplayGain loudness calculation (#2893)

* Fix OpenSubsonic ReplayGain loudness calculation

The OpenSubsonic provider was passing raw ReplayGain gain values
(in dB) directly to set_loudness(), but set_loudness() expects
integrated loudness values (in LUFS).

This caused tracks with ReplayGain tags to show incorrect loudness
values when accessed via OpenSubsonic/Navidrome. For example, a
quiet track with +0.39 dB gain was being stored as 0.39 LUFS
instead of -18.39 LUFS, resulting in massive gain reduction
(-17.39 dB) instead of a small boost.

Fixed by converting ReplayGain values to loudness before storing:
  Loudness (LUFS) = -18 - Gain (dB)

This matches the ReplayGain 2.0 specification and how the
filesystem provider handles ReplayGain tags.

Only affects users connecting to Navidrome/Subsonic servers.
Users with local filesystem music were not affected.

* Remove unnecessary comment

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Co-authored-by: Claude <noreply@anthropic.com>