Optimize 'Leaderboard' query with caching

[gabito1451]

Context

Gamification without slowdown.

Problem

Leaderboard scans 'Total Investments' across the whole DB on every hit.

Suggested Execution

• Implement a 'TopInvestors' cache in Redis.
• Updates only every 1 hour or when a massive investment occurs.

Acceptance Criteria

• Scalable leaderboard for millions of users.
• Instant response time.

References

• backend/src/reputation/leaderboard.service.ts