refactor(channel): remove dead poll_interval_secs from EmailConfig

Field is unused since the IMAP polling loop was replaced with IDLE.
Serde ignores unknown fields on deserialization, so existing configs
with the key set will continue to work without error.

Also add two focused tests for idle_timeout_secs: explicit
deserialization and propagation into EmailChannel.

src/channels/email_channel.rs

@@ -63,9 +63,6 @@ pub struct EmailConfig {
     /// RFC 2177 recommends clients restart IDLE every 29 minutes
     #[serde(default = "default_idle_timeout")]
     pub idle_timeout_secs: u64,
-    /// Fallback poll interval in seconds when IDLE is not supported (default: 60)
-    #[serde(default = "default_poll_interval")]
-    pub poll_interval_secs: u64,
     /// Allowed sender addresses/domains (empty = deny all, ["*"] = allow all)
     #[serde(default)]
     pub allowed_senders: Vec<String>,
@@ -83,9 +80,6 @@ fn default_imap_folder() -> String {
 fn default_idle_timeout() -> u64 {
     1740 // 29 minutes per RFC 2177
 }
-fn default_poll_interval() -> u64 {
-    60
-}
 fn default_true() -> bool {
     true
 }
@@ -103,7 +97,6 @@ impl Default for EmailConfig {
             password: String::new(),
             from_address: String::new(),
             idle_timeout_secs: default_idle_timeout(),
-            poll_interval_secs: default_poll_interval(),
             allowed_senders: Vec::new(),
         }
     }
@@ -603,7 +596,6 @@ mod tests {
         assert_eq!(config.password, "");
         assert_eq!(config.from_address, "");
         assert_eq!(config.idle_timeout_secs, 1740);
-        assert_eq!(config.poll_interval_secs, 60);
         assert!(config.allowed_senders.is_empty());
     }
 
@@ -620,7 +612,6 @@ mod tests {
             password: "pass123".to_string(),
             from_address: "bot@example.com".to_string(),
             idle_timeout_secs: 1200,
-            poll_interval_secs: 30,
             allowed_senders: vec!["allowed@example.com".to_string()],
         };
         assert_eq!(config.imap_host, "imap.example.com");
@@ -641,7 +632,6 @@ mod tests {
             password: "secret".to_string(),
             from_address: "bot@test.com".to_string(),
             idle_timeout_secs: 1740,
-            poll_interval_secs: 120,
             allowed_senders: vec!["*".to_string()],
         };
         let cloned = config.clone();
@@ -867,11 +857,6 @@ mod tests {
         assert_eq!(default_imap_folder(), "INBOX");
     }
 
-    #[test]
-    fn default_poll_interval_returns_60() {
-        assert_eq!(default_poll_interval(), 60);
-    }
-
     #[test]
     fn default_true_returns_true() {
         assert!(default_true());
@@ -892,7 +877,6 @@ mod tests {
             password: "password123".to_string(),
             from_address: "bot@example.com".to_string(),
             idle_timeout_secs: 1740,
-            poll_interval_secs: 30,
             allowed_senders: vec!["allowed@example.com".to_string()],
         };
 
@@ -919,7 +903,30 @@ mod tests {
         assert_eq!(config.smtp_port, 465); // default
         assert!(config.smtp_tls); // default
         assert_eq!(config.idle_timeout_secs, 1740); // default
-        assert_eq!(config.poll_interval_secs, 60); // default
+    }
+
+    #[test]
+    fn idle_timeout_deserializes_explicit_value() {
+        let json = r#"{
+            "imap_host": "imap.test.com",
+            "smtp_host": "smtp.test.com",
+            "username": "user",
+            "password": "pass",
+            "from_address": "bot@test.com",
+            "idle_timeout_secs": 900
+        }"#;
+        let config: EmailConfig = serde_json::from_str(json).unwrap();
+        assert_eq!(config.idle_timeout_secs, 900);
+    }
+
+    #[test]
+    fn idle_timeout_propagates_to_channel() {
+        let config = EmailConfig {
+            idle_timeout_secs: 600,
+            ..Default::default()
+        };
+        let channel = EmailChannel::new(config);
+        assert_eq!(channel.config.idle_timeout_secs, 600);
     }
 
     #[test]