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.

Start by reviewing the playlist generation service and the current behavior described in the issue. Break repetitive playlists, meaningless ordering, limited song characteristics, and lack of new-track discovery into separate issues, then propose a prioritization; done means the scopes and ordering are documented as actionable follow-up issues.

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