Add missing files from P1-040/P1-041 implementation:
- CheckoutRateLimitException for 429 responses when rate limit exceeded
- FraudAssessment data object for fraud scoring results
- FraudService for velocity checks and Stripe Radar integration
- Register services in Boot.php
- Add fraud detection configuration in config.php
- Add CouponServiceTest for input sanitisation
The CheckoutRateLimiter (already tracked) is now properly integrated with
the exception handling, and the FraudService provides defence-in-depth
with velocity-based and geo-anomaly detection.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
