Cron Cast Thumbnails

A frame for the casts that still have no picture (a native video embed carries none, or the stream gave no frame at ingest): a few per run, from their stream, newest first, inside a wall-clock budget that fits the job's attempt deadline (320 s; see farcaster_cast_thumbnail_service for the arithmetic). Its own job, so ffmpeg's slowness on an unreadable or stalled stream can never fail the webhook reconcile. Returns the counts for the log; a run that finds nothing to do is a success.