[Backend] Rewards Leaderboard API

[Olowodarey]

📋 Description

Create competitive leaderboards for various reward metrics with privacy controls.

Acceptance Criteria

• GET /rewards/leaderboard/points for top point earners
• GET /rewards/leaderboard/streaks for longest streaks
• GET /rewards/leaderboard/savings for highest savers
• Support time periods: weekly, monthly, all-time
• Privacy option to hide from leaderboard
• Show user's rank even if not in top 100
• Pagination support

Tasks

• Create leaderboard endpoints
• Implement ranking system
• Add privacy controls
• Support time periods
• Add pagination
• Write tests