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/appsettings.json

@@ -9,7 +9,7 @@
     "ExpiryInHours": 24
   },
   "SilverPay": {
-    "BaseUrl": "http://31.97.57.205:8001",
+    "BaseUrl": "http://10.0.0.52:8001",
     "ApiKey": "sp_live_key_2025_production",
     "WebhookSecret": "webhook_secret_2025",
     "DefaultWebhookUrl": "http://localhost:8080/api/orders/payments/webhook",