feat(leaderboard): add live Codeforces and CodeChef sync, Redis caching and cooldown API

[Shubham15986]

Description

This PR implements the college competitive programming leaderboard backend for NIT Durgapur, pulling live standings from Codeforces and CodeChef with Redis caching.

Resolves #163

Key Changes

• Live Scrapers:
  • Codeforces organization sync targeting NIT Durgapur (https://codeforces.com/ratings/organization/785) capturing handles, ratings, rank titles, contest counts, and active status.
  • CodeChef institution sync with CSRF and pagination support capturing ratings, stars, global ranks, and active status.
• Data Models & Normalization:
  • Symmetrical fields for both platforms with is_active platform flags.
  • Symmetrical ranking calculations and dynamic filtering (active_only / show_inactive).
• Performance & Redis Caching:
  • Redis cache integration with 300s TTL per filter configuration (~4.7ms response times).
  • Dedicated refresh endpoint (POST /api/leaderboard/refresh/) with a strict 5-minute cooldown timer to prevent rate limits.
  • Cooldown query endpoint (GET /api/leaderboard/cooldown/).
• Management Command:
  • python manage.py sync_leaderboard for scheduled or manual sync of college standings.

Endpoints Added

• GET /api/leaderboard/?platform=codeforces|codechef&search=...&active_only=true|false
• POST /api/leaderboard/refresh/
• GET /api/leaderboard/cooldown/

Deployment Notes

After merging, run:

python manage.py migrate
python manage.py sync_leaderboard