Podcast & Podcast DB updates

[hasezoey]

This PR refactors the podcast modules to be more maintainable and consistent with the other database, some more details:

• move the taskpool outside to be re-used outside of just podcasts
• change many panics to results
• extract many structs (and their impls) out into separate modules
• fix doc-comments
• update podcast db to be more maintainable (similar to #338)
• set user_version for the podcast db and use that instead for versioning (like the other database)
• abstract every database table to their own module (similar to #338)
• general cleanup (remove indentation, unnecessary explicit types, clippy lints)
• extend the content_type list