Start using classes for Radio playlists and stations

[C0rn3j]

Radio playlists used multinested list of dictionaries, converted it to classes instead.

radio_playlists: list[dict[str, int | str | list[dict[str, str]]]]

radio_playlists: list[RadioPlaylist] 

• Exit if we fail to run migrations
• Requires a database bump (now 70)
• Corrects typing for RadioPlaylist variable in db_migrations
• Fixes gensokyoradio.net fallback URL (I thought I did that ages ago??)
• Stop setting scroll == 0 on Stations, it was there in one place in the code, makes no sense to me and seems like a mistake, should be set on Playlist instead?

TODO:

• Editing seems to lose site URL - not fault of this PR, it gets nulled on edit by design for some reason - at least added a check there even though there's no way to set site URL manually, we should probably expose all parameters in a later PR
• Needs testing