refactor(channel): accept SendMessage struct in Channel::send()

Refactor the Channel trait to accept a SendMessage struct instead of
separate message and recipient string parameters. This enables passing
additional metadata like email subjects.

Changes:
- Add SendMessage struct with content, recipient, and optional subject
- Update Channel::send() signature to accept &SendMessage
- Update all 12 channel implementations
- Update call sites in channels/mod.rs and gateway/mod.rs

Subject field usage:
- Email: uses subject for email subject line
- DingTalk: uses subject as markdown message title
- All others: ignore subject (no native platform support)

src/channels/whatsapp.rs

@@ -1,4 +1,4 @@
-use super::traits::{Channel, ChannelMessage};
+use super::traits::{Channel, ChannelMessage, SendMessage};
 use async_trait::async_trait;
 use uuid::Uuid;
 
@@ -139,7 +139,7 @@ impl Channel for WhatsAppChannel {
         "whatsapp"
     }
 
-    async fn send(&self, message: &str, recipient: &str) -> anyhow::Result<()> {
+    async fn send(&self, message: &SendMessage) -> anyhow::Result<()> {
         // WhatsApp Cloud API: POST to /v18.0/{phone_number_id}/messages
         let url = format!(
             "https://graph.facebook.com/v18.0/{}/messages",
@@ -147,7 +147,10 @@ impl Channel for WhatsAppChannel {
         );
 
         // Normalize recipient (remove leading + if present for API)
-        let to = recipient.strip_prefix('+').unwrap_or(recipient);
+        let to = message
+            .recipient
+            .strip_prefix('+')
+            .unwrap_or(&message.recipient);
 
         let body = serde_json::json!({
             "messaging_product": "whatsapp",
@@ -156,7 +159,7 @@ impl Channel for WhatsAppChannel {
             "type": "text",
             "text": {
                 "preview_url": false,
-                "body": message
+                "body": message.content
             }
         });