Video thumbnails are now generated too (recursively, along with images) on the `thumb`
command. The poster frame is extracted at 25% of the duration (instead of a fixed 10s
seek).
Thumbnails are now kept in cache indefinitely.
This is a huge configuration refactoring.
The configuration system now uses Pydantic settings, with better validation, better
support for environment variables and other goodies.
The main config files now use TOML instead of INI, this allow for better handling of
complex types, notably dict.
Plugins, components and frontends can declare their config settings in a class which will
be imported dynamically.
`render_OPTIONS` is a method inherited from Twisted, name has been put to lowercase
following massive refactoring, resulting in the method not being called anymore. This
patch fixes it.
