Remote node and Emby/Jellyfin webhook fixes

Fixes

• Remote nodes: a node that had just started could fail its first two tasks if they arrived at the same moment. That failure could then freeze the app's Intro & Credits worker and stall every Plex marker and loudness write for hours with "Another task is still using this Plex database". Both are fixed.
• Remote nodes: an upgraded file that a node still sees at its old size no longer pauses the node. A paused node now re-checks itself every 5 minutes instead of waiting for you to start a check.
• Emby and Jellyfin: Recently Added scans found nothing on servers with two or more libraries that have Previews turned on. Each library is now asked on its own. (#432)
• Emby and Jellyfin: the webhook setup steps now point at the webhook secret (Automation → Triggers → Settings), not the sign-in token, which caused every webhook to be rejected. (#432)
• Intro & Credits: files with no IntroDB or SkipDB entry are re-checked online over 14 to 28 days instead of all at once, so a full library check no longer slows to a crawl every two weeks.

Other

• The Docker image is labelled as Media Preview Generator with a real build date, instead of inheriting linuxserver/ffmpeg's labels. (#428)