Improve channel discovery/logging, pruning tracking, and frontend log UX

[FizzyGalacticus]

What changed

A solid round of quality-of-life updates across backend + frontend, mostly around channel workflow and visibility.

Highlights:

• Channel checks are more reliable and handle cutoff/retention behavior more cleanly.
• Pruning/download tracking is tighter, so we avoid re-download loops.
• Channel-scoped logs are much more useful now (feed checks, download attempts/results, prune activity).
• Logs API + UI filtering got cleaned up so it’s easier to follow what happened for a specific channel.
• Channels/videos UI got a broader polish pass, especially around logs and list interactions.
• Docs were updated to match the new behavior.