Display top contributors by reputation.

Acceptance Criteria:

• Create /leaderboard route
• Show top 100 users by reputation
• Add time period filters: All-Time, This Year, This Month, This Week
• Display rank, avatar, username, reputation, badge count
• Highlight current user's position
• Create leaderboard caching with scheduled refresh
• Add category tabs: Overall, Reports, Vulnerabilities, Community (comments)
• Show position change indicators (↑↓)