[G7] Leaderboard: global and friends tabs, period filters

Ranking by XP. Gated behind SOCIAL_ENABLED, like every social surface.

Depends on: [G1] XP ledger (materialised counter + ledger for period queries).

Scope

• Two tabs: global and friends.
• Period filters: month and year for the MVP. Recomputable from the ledger, so no snapshot table is needed yet.
• Everyone participates — no opt-in. Ghosts (Figurants) are excluded.
• Performance: rank on the materialised counter on User, not by summing the ledger at read time.

The decision to make here

PRIVATE accounts do appear in the global leaderboard. Rationale: a private profile keeps all its actual media data hidden, and the ranking exposes nothing about consumption. What is still open is the presentation — a private account seen by a non-friend may deserve a neutral avatar rather than its real picture, while keeping its pseudo. Settle it in this ticket, and reflect whatever is decided in the privacy policy.

Related

Penalties and score integrity are [B8], not this ticket. Leaderboard seasons with an archived podium are [B5].