🔒 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/ISystemSettingsService.cs

@@ -0,0 +1,13 @@
+namespace LittleShop.Services;
+
+public interface ISystemSettingsService
+{
+    Task<string?> GetSettingAsync(string key);
+    Task<T?> GetSettingAsync<T>(string key, T? defaultValue = default);
+    Task SetSettingAsync(string key, string value, string? description = null);
+    Task SetSettingAsync<T>(string key, T value, string? description = null);
+    Task<bool> DeleteSettingAsync(string key);
+    Task<Dictionary<string, string>> GetAllSettingsAsync();
+    Task<bool> IsTestCurrencyEnabledAsync(string currency);
+    Task SetTestCurrencyEnabledAsync(string currency, bool enabled);
+}