Jellyfin Integration and Subsonic Improvements

New

• Add Jellyfin as a source #15

Changed

• Changed polling backoff behavior to use time-based backoff instead of interval count
• Implemented in-memory "recently played" logic for generic Source to better support scrobbling from sources without their own "recently played" source of truth or scrobbling behavior. Used for Subsonic to make scrobbling behavior more stable and on Jellyfin integration.

Fixed

• Fixed issue where Last.fm "now playing" entry returned from "recently played" broke last.fm scrobbling #22