Implement critical security fixes from code review

LittleShop/Services/ConfigurationValidationService.cs

@@ -29,8 +29,16 @@ public class ConfigurationValidationService
     {
         _logger.LogInformation("🔍 Validating application configuration...");
 
-        // Temporarily disabled for testing SilverPay settings page
-        // ValidateJwtConfiguration();
+        // JWT validation is critical in production, optional in development/testing
+        if (_environment.IsProduction() || !string.IsNullOrEmpty(_configuration["Jwt:Key"]))
+        {
+            ValidateJwtConfiguration();
+        }
+        else if (_environment.IsDevelopment())
+        {
+            _logger.LogWarning("⚠️ JWT validation skipped in development. Configure Jwt:Key for production readiness.");
+        }
+
         ValidateSilverPayConfiguration();
         ValidateProductionSafeguards();
         ValidateEnvironmentConfiguration();