- The tag parser's cache is now cleared between indexing runs, preventing stale data from being used
 - Multi-value tag fields (genres, all artists) are now properly ingested in the tag value cache
 - Cleaning up removed index records now properly handles the case where only a subset of the records for multi-value tags need to be deleted.
 - Synthesizing missing tag values is now done in the tag parser instead of TrackTags, which resolves some issues with multi-value tag callbacks from libtags not being handled properly

These fixes seem to address all the issues with stale index records we were able to repro (including the issues in https://codeberg.org/cool-tech-zone/tangara-fw/issues/191), but if you've got any more cases with consistent repros then lmk!

Co-authored-by: ailurux <ailuruxx@gmail.com>
Reviewed-on: https://codeberg.org/cool-tech-zone/tangara-fw/pulls/193

We still mostly use the singular 'Artist' tag for e.g. displaying a nice
name in the now playing screen, but where a track has an 'ARTISTS=' tag,
we'll split by semicolon and then use the resulting list to populate an
index of tracks by artist

Initially set based on filepath, or by genre if the filepath doesn't
match one of our presets

This also has the effect of breaking up the enormous 'updateIndexes'
method into one call per file, which means database updates also no
longer monopolise a single background task for their entire duration.

avg. time per new file is now <140ms for a completely fresh database,
which is pretty good i think!