feat: Add automated achievement system triggered by user milestones

[amaydixit11]

Feature Request

Problem

The Achievement model (models.py) exists but achievements can only be unlocked via manual API calls to /api/achievements/unlock. There's no automated system that awards achievements based on actual user behavior.

Proposed Solution

Implement a rule-based achievement engine:

1. First Codeforces submission
2. First LeetCode problem solved
3. GitHub contributor streak (7, 30, 100 day streaks)
4. 100/500/1000 total problems solved milestone
5. First organization join
6. Top 10 in any platform leaderboard

Technical Notes

The /api/achievements/unlock endpoint already exists. Add a periodic check (Celery beat task) or real-time trigger on data refresh that evaluates achievement conditions and calls the existing unlock endpoint.