Fix SilverPay payment integration JSON serialization

- Changed JSON naming policy from CamelCase to SnakeCaseLower for SilverPay API compatibility
- Fixed field name from 'fiat_amount' to 'amount' in request body
- Used unique payment ID instead of order ID to avoid duplicate external_id conflicts
- Modified SilverPayApiResponse to handle string amounts from API
- Added [JsonIgnore] attributes to computed properties to prevent JSON serialization conflicts
- Fixed test compilation errors (mock service and enum casting issues)
- Updated SilverPay endpoint to http://10.0.0.52:8001/

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop/Services/CryptoPaymentService.cs

@@ -57,8 +57,11 @@ public class CryptoPaymentService : ICryptoPaymentService
             // Use SilverPAY
             _logger.LogInformation("Creating SilverPAY order for {Currency}", currency);
 
+            // Generate payment ID first to use as external_id
+            var paymentId = Guid.NewGuid();
+
             var silverPayOrder = await _silverPayService.CreateOrderAsync(
-                order.Id.ToString(),
+                paymentId.ToString(),  // Use unique payment ID instead of order ID
                 order.TotalAmount,
                 currency,
                 $"Order #{order.Id} - {order.Items.Count} items",
@@ -67,7 +70,7 @@ public class CryptoPaymentService : ICryptoPaymentService
 
             var cryptoPayment = new CryptoPayment
             {
-                Id = Guid.NewGuid(),
+                Id = paymentId,  // Use the same payment ID
                 OrderId = orderId,
                 Currency = currency,
                 WalletAddress = silverPayOrder.PaymentAddress,