Add support for Hardcover

[dankozlowski]

A FOSS alternative to goodreads is https://hardcover.app. It would be nice to be able to import from readings lists created there.

I (ticket author) can take a look at a first-pass implementation.

[StirlingMouse]

Does hardcover have an API or RSS to programatically get a list? I haven't used it but aren't finding an obvious way how to do this for it. But it it does, it could absolutely be added!

I just added notion lists/dbs so the code is now much better structured to handle multiple sources so feel free to take a stab at it. You'd be interested in the server/src/lists directory as well as adding new config options in the server/src/config.rs file.