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.Tests/Integration/OrdersWithVariantsTests.cs

@@ -269,7 +269,7 @@ public class OrdersWithVariantsTests : IClassFixture<TestWebApplicationFactory>
             Price = 15.00m,
             CategoryId = category.Id,
             Weight = 1.0m,
-            WeightUnit = (int)LittleShop.Enums.ProductWeightUnit.Kilograms
+            WeightUnit = LittleShop.Enums.ProductWeightUnit.Kilograms
         };
 
         var response = await _client.PostAsJsonAsync("/api/admin/products", productDto);