RequestCast 1.8.3

Livestream requests now play immediately instead of hanging in the queue, and the
livestream title is pushed to Liquidsoap.

Livestreams fail fast and never retry. A livestream that is queued behind an
older job (or that can't be relayed) used to sit in the queue as "waiting for the
downloader" forever — the downloader kept trying to grab a stream that never ends.
Livestream downloads now raise a distinct LiveStreamError that fails the job
immediately with a clear message and is excluded from automatic retries and
requeues, so nothing blocks the queue.

Livestream metadata is pushed to air. When a livestream is relayed, the
stream's title and artist are pushed to Liquidsoap via custom_metadata.insert
right after the harbor switch, and refreshed periodically while it plays — so the
on-air display shows the actual stream title instead of a stale AutoDJ track.
Configured with azuracast_live_metadata_url (the telnet/API endpoint) and
azuracast_live_metadata_key, or on a server
REQUESTCAST_AZURACAST_LIVE_METADATA_URL / REQUESTCAST_AZURACAST_LIVE_METADATA_KEY.