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.Development.json

@@ -3,7 +3,7 @@
     "DefaultConnection": "Data Source=littleshop-dev.db"
   },
   "Jwt": {
-    "Key": "DEV_8aiNFkRrOao7/vleviWM8EP5800dMOh2hlaKGJoQOQvaxxOVHM3eLAb3+5KN8EcjKZKREHttGKUfvtQrV3ZM4A==",
+    "Key": "DEVELOPMENT_USE_DOTNET_USER_SECRETS_OR_ENV_VAR",
     "Issuer": "LittleShop-Dev",
     "Audience": "LittleShop-Dev",
     "ExpiryInHours": 2

LittleShop/appsettings.json

@@ -3,7 +3,7 @@
     "DefaultConnection": "Data Source=littleshop.db"
   },
   "Jwt": {
-    "Key": "9xKmN3pQwR7vYzH4bFtJ8sLcE2nW6aVgDhU5kXmP1oZiAqBjCrTy0MxSfGdIlPeWuO",
+    "Key": "",
     "Issuer": "LittleShop",
     "Audience": "LittleShop",
     "ExpiryInHours": 24