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/

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