Adds cleanup_audio_cache() to gateway/platforms/base.py and wires it into
_start_gateway_housekeeping() in gateway/run.py, matching the existing
image/document cache cleanup pattern.
Re-implements the fix proposed in #16473, which targeted the now-deprecated
_start_cron_ticker() and has since stalled with a merge conflict against
main. _start_gateway_housekeeping()'s docstring already claimed audio
cleanup ("prunes the image/audio/document cache"); this makes the code
match it.
Co-authored-by: nftpoetrist <264138787+nftpoetrist@users.noreply.github.com>
