🔒 SECURITY: Emergency fixes and hardening

EMERGENCY FIXES:
✅ DELETE MockSilverPayService.cs - removed fake payment system
✅ REMOVE mock service registration - no fake payments possible
✅ GENERATE new JWT secret - replaced hardcoded key
✅ FIX HttpClient disposal - proper resource management

SECURITY HARDENING:
✅ ADD production guards - prevent mock services in production
✅ CREATE environment configs - separate dev/prod settings
✅ ADD config validation - fail fast on misconfiguration

IMPACT:
- Mock payment system completely eliminated
- JWT authentication now uses secure keys
- Production deployment now validated on startup
- Resource leaks fixed in TeleBot currency API

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

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

LittleShop/Services/ISilverPayService.cs

@@ -42,6 +42,12 @@ public interface ISilverPayService
     /// <param name="fiatCurrency">Fiat currency (GBP, USD, EUR)</param>
     /// <returns>Current exchange rate</returns>
     Task<decimal?> GetExchangeRateAsync(string cryptoCurrency, string fiatCurrency = "GBP");
+
+    /// <summary>
+    /// Get list of supported cryptocurrencies from SilverPAY
+    /// </summary>
+    /// <returns>List of supported currency codes</returns>
+    Task<List<string>> GetSupportedCurrenciesAsync();
 }
 
 /// <summary>