[Backend] Rewards Challenges API

[Olowodarey]

📋 Description

Implement time-bound challenges that reward users for completing specific actions.

Acceptance Criteria

• GET /rewards/challenges/active to list current challenges
• POST /rewards/challenges/:id/join to opt into challenge
• GET /rewards/challenges/:id/progress to track completion
• Automatic reward distribution on completion
• Challenge types: deposit streaks, goal creation, referrals
• Admin API to create and manage challenges
• Leaderboard for each challenge

Tasks

• Create challenge endpoints
• Implement progress tracking
• Add auto-reward distribution
• Create admin management
• Add challenge leaderboards
• Write tests