T4-042: Caching layer — Redis for leaderboards and challenge data

Track: 4 — Telegram Mini App
Section: Phase E: Infrastructure & Polish (18h)
Status: [ ] Todo
Repo: backend-api
Branch: feature/T4-042-redis-caching-layer
Estimated Hours: 4h

Done When

Redis cache for: challenge leaderboards (TTL 5min), user profiles (TTL 1h), challenge list (TTL 10min). Reduces DB load

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