Add leaderboard query: top users by wrap count

[Calebux]

Problem

No way to query top users by number of wraps. Useful for gamification and frontend leaderboards.

Considerations

Soroban storage doesn't support range queries. Options:

1. Off-chain indexing — index events, build leaderboard in backend (recommended)
2. On-chain sorted list — maintain a top-N list on-chain (expensive)

Acceptance Criteria

• Decision: on-chain vs off-chain leaderboard
• If off-chain: document event schema for indexers
• If on-chain: implement top-N tracking with bounded storage

[Ericoluchukwu]

@Ericoluchukwu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issues because I'm a developer

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Ericoluchukwu to this issue.