[B3] Recaps and comparatives: yearly Rewind, monthly retro, anonymous percentiles

Backlog, explicitly late.

Ideas parked here

• Yearly Rewind. Your year in numbers, top rated, the genre that exploded, longest streak, plus a shareable image. High potential, but it demands real design work and, above all, accounts with several months of history — which does not exist yet. Premium candidate (see [B9]). Should be one of the last features, not one of the first.
• Monthly retrospective. Lighter version, pushed through the existing NotificationDigestService infrastructure. Lower enthusiasm, but it is the incremental path toward the Rewind.
• Anonymous percentiles — "you are in the top 10% of documentary watchers". All the satisfaction of comparison, none of the leaderboard's effects: no name, no race. Complements [G7] rather than duplicating it.

Existing groundwork

The stats/ module already computes most of the aggregates: video-temporal, rating-distribution, decade, cross-domain-totals.