refactor(channel): replace hand-rolled IMAP with async-imap IDLE

Replace the blocking, poll-based IMAP client with async-imap and
IMAP IDLE (RFC 2177) for instant push delivery. Key changes:

- Add async-imap dependency with tokio runtime feature
- Rewrite connect/fetch/listen paths to fully async using tokio TLS
- Implement IDLE loop with exponential backoff reconnection (1s–60s cap)
- Add idle_timeout_secs config field (default 1740s per RFC 2177)
- Convert health_check to async connect-and-logout with 10s timeout
- Update affected tests from sync to #[tokio::test]

SMTP send path, allowlist enforcement, and Channel trait contract
are unchanged.

Cargo.toml

@@ -99,13 +99,16 @@ futures-util = { version = "0.3", default-features = false, features = ["sink"]
 futures = "0.3"
 regex = "1.10"
 hostname = "0.4.2"
-lettre = { version = "0.11.19", default-features = false, features = ["builder", "smtp-transport", "rustls-tls"] }
-mail-parser = "0.11.2"
 rustls = "0.23"
 rustls-pki-types = "1.14.0"
 tokio-rustls = "0.26.4"
 webpki-roots = "1.0.6"
 
+# email
+lettre = { version = "0.11.19", default-features = false, features = ["builder", "smtp-transport", "rustls-tls"] }
+mail-parser = "0.11.2"
+async-imap = { version = "0.11",features = ["runtime-tokio"], default-features = false }
+
 # HTTP server (gateway) — replaces raw TCP for proper HTTP/1.1 compliance
 axum = { version = "0.8", default-features = false, features = ["http1", "json", "tokio", "query", "ws"] }
 tower = { version = "0.5", default-features = false }