[B7] Social adjacent: taste affinity, side-by-side ratings, following a list

Backlog. Social features that came out of the gamification discussion but are not gamification themselves. All gated behind SOCIAL_ENABLED.

Ideas parked here

• Taste affinity score between two users, computed from ratings on works they have both rated. Sticky and genuinely useful, and non-competitive — but expensive to compute well. Possibly a premium-profile stat (see [B9]).
• Side-by-side ratings on a work page: your friends' ratings next to yours ("you 8 · Marc 4 · Julie 9"). Simple, and it makes the social layer feel alive.
• Following a list. Does not exist today — a list can be shared but not followed. It is a prerequisite for any "list followed by N people" achievement, and useful on its own.

Note

Review is already the single source of truth for ratings (ReviewService.getRatings), which is what makes the first two feasible at all.