Add ingest health monitor & feed health status

[changesbyjames]

Currently, if an ingest is unhealthy (source feed is down, streamlink has broken, ffmpeg has broken, screenshots are not being made/are blank, etc) we have no way of knowing until it's manually reset.

• Add a watchdog mechanism
• Report issues
• Exit when unhealthy (so docker can restart)
• Add a heartbeat
• Check feed health before creating snapshots
• Fallback to hosted twitch feed if other feeds are down