Scope Work - Improve Playlist Generation Service

# Background
The service that generates playlists is _very_ simple. It simply...

1) Sets some "Liked Song" threshold
2) Finds the Liked Songs for each Listener
3) Chooses the Intersection or Difference depending on the Preference Type requested
- e.g. provide the Intersection if a playlist that both listeners like is requested

This leaves a lot of room for improvement.

## Frictions
- Playlists are repetitive
- Generating the same playlist type for the same listeners provides the exact same playlist
- Playlists order doesn't have meaning
- We provide songs back in whatever order
- Playlists only consider the liked song threshold
- They don't consider things like the songs' characteristics
- There is no new-track discovery
- We don't introduce any new songs into the playlist

# Goal - Scope Work to Improve Playlist Generate Service
Create new Issues to break this work into separate pieces we can make progress on. Propose a prioritization of these scopes.