[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.
• Library comparison with a friend: "34 titles in common; they gave 9/10 to three films you haven't seen." Entries, ratings and the visibility service are already there, so only what the other person shares is shown (social/profile.service.ts, social/visibility.service.ts). Distinct from "what to watch together", which crosses both watchlists.

Note

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