feat(backend): add loyalty constants and first-payment guard

Extract LoyaltyStampCost and add LoyaltyDiscountPercent constant. Add first-payment guard to ApplyLoyaltyRedemption — reject if a non-discount payment already exists on the booking.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/refunds.go

@@ -255,7 +255,7 @@ func ProcessCancellationRefund(
 			if err != nil {
 				log.Printf("Failed to refund loyalty stamps for booking %s: %v", bookingID, err)
 			} else {
-				log.Printf("Refunded 10 loyalty stamps to user %s after cancellation of booking %s", bookingUserID, bookingID)
+				log.Printf("Refunded %d loyalty stamps to user %s after cancellation of booking %s", LoyaltyStampCost, bookingUserID, bookingID)
 			}
 		}
 	}