Ability to configure fallback thumbnail per channel synchronization

### Describe the problem to be solved

Not all sites that `yt-dlp` [supports](https://github.com/yt-dlp/yt-dlp/blob/master/supportedsites.md) exporting a thumbnail, yielding a default one to be picked. This global default thumbnail however can't reflect channel specific branding if the instance is used for multiple channels.

### Describe the solution you would like

PeerTube allowing a fallback thumbnail to be configured per channel synchronization.

The issue names no files, tests, or entry points. Start by tracing the existing channel-synchronization flow and global fallback-thumbnail configuration; determine how a per-channel setting should be represented and verify that a synchronized channel without an exported thumbnail uses its configured image.

Written by the indexing model from the issue text.