T4-041: Rate limiting for mini app API endpoints

Track: 4 — Telegram Mini App
Section: Phase E: Infrastructure & Polish (18h)
Status: [ ] Todo
Repo: backend-api
Branch: feature/T4-041-miniapp-rate-limiting
Estimated Hours: 3h

Done When

Rate limit mini app endpoints: 60 req/min per user. Anti-abuse for challenge joins. Return 429 with retry_after

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