fix: address PR #37 review issues

- Add missing EmailConfig struct with serde derives and defaults
- Register email_channel module in mod.rs with exports
- Fix IMAP tag reuse (RFC 3501 violation) using incrementing counter
- Fix email sender validation logic (clearer domain vs full email matching)
- Fix mail_parser API usage (MessageParser::default().parse())
- Fix WhatsApp allowlist matching (normalize phone numbers)
- Fix WhatsApp health_check (don't treat 404 as healthy)
- Fix WhatsApp listen() to keep task alive (prevent channel bus closing)
- Add missing dependencies: lettre, mail-parser, rustls-pki-types, tokio-rustls, webpki-roots
- Remove unused imports

All 665 tests pass.

Cargo.toml

@@ -58,6 +58,11 @@ console = "0.15"
 tokio-tungstenite = { version = "0.24", features = ["rustls-tls-webpki-roots"] }
 futures-util = { version = "0.3", default-features = false, features = ["sink"] }
 hostname = "0.4.2"
+lettre = { version = "0.11.19", features = ["smtp-transport", "rustls-tls"] }
+mail-parser = "0.11.2"
+rustls-pki-types = "1.14.0"
+tokio-rustls = "0.26.4"
+webpki-roots = "1.0.6"
 
 [profile.release]
 opt-level = "z"      # Optimize for size