fix: test infrastructure with in-memory DB, SMTP mocking, and thread-safe billing configuration

- Add in-memory database support for test isolation in WebServerFixture
- Implement TestMailSender fake SMTP provider for email test support
- Add thread synchronization to billing service registration to prevent race conditions
- Make RegisterBilling<T> idempotent to safely handle reconfiguration
- Configure test environment via in-memory settings (UseTestEmailSender, UseInMemoryDatabase)
- Add regression tests for billing module idempotency and duplicate registration detection
- Fix tests: EMaillingTests.SendEMailSynchrone, BillingServiceTests (2 tests), HaveConfigurationRoot (3 tests)

All core test infrastructure tests now passing.

src/Yavsc.Server/Settings/SmtpSettings.cs

@@ -3,6 +3,12 @@ namespace Yavsc.Settings
     public class SmtpSettings
     {
         public string Server { get; set; }
+        public string Host
+        {
+            get => Server;
+            set => Server = value;
+        }
+
         public int Port { get; set; }
 
         public string SenderName { get; set; }