Fix playlist stopping between tracks; add retry logic to getNextTrack

[ElFishi]

Fix playlist stopping between tracks; add retry logic to getNextTrack

This PR fixes a bug that caused playlist playback to stop between tracks instead of advancing automatically, and adds automatic retry logic to make the player resilient against the transient yt-dlp failures that triggered it.

Bug

When yt-dlp resolves a track's stream URL, YouTube occasionally returns only format 18 — a legacy muxed 360p mp4 — instead of the expected audio-only streams (opus/ogg/aac). Since format 18 is not present in the @audioId or @videoId lookup tables, _selectTracks returns an empty list, _getNextTrack finds no matching track and calls errorCb, causing LMS to stop playback entirely. Because the track is not removed from the playlist, pressing play manually succeeds — typically because yt-dlp returns proper audio streams on the second attempt.

A secondary bug existed in _selectTracks: in the audio loop, $track->{_id} = $item was executed after the push and unconditionally, meaning it ran even when no matching track was found (leaving $track undef), and any pushed track had _id unset at the point it was pushed.

Changes

getNextTrack now accepts an optional $_retries parameter, defaulting to 3. On any failure — yt-dlp parse error, no matching tracks, or stream initialisation failure from _getNextTrack — a $retryOrFail closure either schedules a fresh attempt via Slim::Utils::Timers::setTimer after a 2-second delay, or calls the real errorCb once all retries are exhausted. The 2-second delay avoids immediately hammering YouTube's CDN and gives the event loop room to breathe. The $retryOrFail closure is passed down as the error callback to _getNextTrack as well, so failures at the stream initialisation level are retried on the same terms.

In _selectTracks, both the audio and video loops are corrected to assign $track->{_id} before pushing, and only when a matching track was actually found.