feat: implement tiered loyalty & rewards program engine #283

[ZuLu0890]

• Add comprehensive loyalty tier system with 5 tiers (Bronze, Silver, Gold, Platinum, Diamond)
• Implement points earning with tier multipliers (10 points per spent)
• Create points redemption system for discount coupons and checkout flow
• Add birthday and anniversary bonus point systems
• Implement automatic tier progression based on lifetime points
• Add tier-based benefits including discounts, free shipping, and exclusive access
• Create comprehensive API endpoints for rewards and loyalty management
• Add full test coverage for rewards and loyalty services
• Include detailed documentation and integration examples
• Enhance existing rewards system with tiered functionality

Acceptance Criteria:
✅ RewardPoints entity linked to User profile
✅ Order.completed event listener for points calculation (10 points per ) ✅ Redemption logic for points to DiscountCoupons
✅ Points redemption integration in Checkout flow
✅ Enhanced tiered loyalty system with progressive benefits

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closes #283