fix: add leaderboard admin moderation

[Victorakinwunmi]

Summary

• add admin-only player removal, score correction, ban, and unban operations
• reset banned players' points and remove them from the leaderboard
• reject future market and referral point awards for banned players
• keep top-list slots and minimum cache consistent after moderation

Tests

• add coverage for removal, score correction, bans, future-award rejection, and admin authorization
• cargo test -p leaderboard (not run: Rust/Cargo is unavailable in the local environment)

[Victorakinwunmi]

Superseded by per-issue PR that references closes #65.