Add Redis caching for leaderboard API queries

[Anuoluwapo25]

Summary

The leaderboard query aggregates token balances across all learners and is one of the most expensive database queries. Caching the result in Redis with a short TTL (60 s) would dramatically reduce DB load.

Proposed Approach

1. On request, check Redis for a leaderboard:top100 key
2. Cache hit → return cached JSON immediately
3. Cache miss → run DB query, store result with 60 s TTL, return

Acceptance Criteria

• Redis client configured in server/src/lib/
• Cache-aside pattern applied to the leaderboard controller
• Cache is invalidated (or TTL respected) when token balances change
• REDIS_URL added to server/.env.example
• Unit test mocks Redis and verifies cache hit/miss paths

Files

• server/src/controllers/
• server/src/lib/