Add comprehensive integration test suite (51 tests)

- Email sending/receiving tests
- SMTP commands: HELO, EHLO, NOOP, RSET, VRFY, QUIT
- TLS/SMTPS and STARTTLS tests
- SMTP AUTH tests
- WebSocket real-time updates tests
- HTTP API tests (GET, DELETE)
- Whitelist filtering tests
- Concurrent sending tests
- Multipart email tests
- Unicode/special characters tests
- Large email body tests
- Edge cases and error handling

Author: jimmystridh

Cargo.toml

@@ -41,6 +41,7 @@ tokio-tungstenite = "0.26"
 futures-util = "0.3"
 urlencoding = "2"
 rustls = "0.23"
+chrono = "0.4"
 
 [lints.clippy]
 pedantic = { level = "warn", priority = -1 }