* fix(smartplaylist): support isMissing/isPresent operators on ReplayGain fields
ReplayGain values are stored in dedicated nullable columns (rg_album_gain,
rg_album_peak, rg_track_gain, rg_track_peak) rather than in the media_file.tags
JSON blob. The isMissing/isPresent operators previously only supported tag and
role fields, causing two failure modes:
1. Using the documented alias names (replaygain_album_gain etc.) from PR #5256:
these got registered as JSON tags from mappings.yaml, so isMissing queried
json_tree(media_file.tags, '$.replaygain_album_gain') which is always empty
-> the playlist matched ALL songs.
2. Using the canonical field names (rgalbumgain etc.): not a tag/role, so SQL
generation returned an error. Because refreshSmartPlaylist deletes old tracks
before regenerating, the abort left the playlist empty.
Fix: add Nullable bool to FieldInfo and mark the four ReplayGain fields. Add
static alias entries (replaygain_album_gain -> rgalbumgain etc.) with
Numeric+Nullable set; because AddTagNames skips names already in the field map,
these static entries take precedence over the mappings.yaml tag registration.
missingExpr now emits IS NULL / IS NOT NULL for nullable column fields instead
of the json_tree lookup.
Fixes#5584
* chore(smartplaylist): address code review feedback
- Simplify the isMissing/isPresent unsupported-field error message,
removing the internal "nullable fields" jargon
- Standardize comments on mappings.yaml (the actual filename)
- Clarify the alias precedence comment in the LookupField test
08fd222791