Store the per-track replay gain a server reports on OpenSubsonic's
`Child.replayGain` so playback can normalise loudness later.
- add trackGain/albumGain/trackPeak/albumPeak (REAL) to the songs table
and the 27 -> 28 migration
- parse the nested `<replayGain>` element, folding the child-element
dispatch into a single `songChild` shared by the search and album paths
- teach the sync upsert to bind Float, which it previously rejected
baseGain and fallbackGain are omitted: the servers that send the object
do not populate them, so they would only ever be null columns.
Nothing applies the values yet; this slice only mirrors them.
