`buildPlaylist` reported `evaluated_at` as `changed` for smart playlists, so a
rename or comment edit was invisible to clients until the next evaluation. A
never-evaluated smart playlist also reported the current time on every call,
which never settled.
Report `updated_at` for every playlist. `refreshCounters` now syncs the stamp it
writes back onto the model, and `refreshSmartPlaylist` reuses it for
`evaluated_at`. `changed` therefore still equals the evaluation time for a
just-evaluated playlist, and `validUntil` stays anchored to it.
Original Subsonic always bumps `changed` on any playlist update, so this also
aligns the behavior with upstream.
a7365e119b