✨ Historical read progress tracking

[aaronleopold]

Resolves #207

This PR splits the read_progresses table into two separate tables:

• active_read_sessions to track the current reading session for a user+book pair
• finished_read_sessions to track the past and completed reading sessions for a user+book pair

There were a lot of fundamental changes to filtering on read progress, as well as displaying progress throughout the UI, so I'll be testing these changes before being comfortable merging. A few behavioral notes:

• A user may have 1 active_read_session per book
• A user may have any number of finished_read_sessions per book
• Marking a book as complete sets the started_at and completed_at timestamps to:
  • The same value (current timestamp) if no active_read_session exists
  • The started_at value from active_read_session and the current timestamp, respectively
• Marking a book as unread will not clear historical record, it will clear the active_read_session if present

I think these changes also open the doors a bit to more time-based statistics features

[aaronleopold]

I've done a bit more testing and fixed a few small issues. I'll try to simulate an upgrade from the current experimental to test the migration, since I wrote bits of it by hand I'd like to be extra cautious to not mess up peoples' read progress