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/matrix.rs

@@ -1,4 +1,4 @@
-use crate::channels::traits::{Channel, ChannelMessage};
+use crate::channels::traits::{Channel, ChannelMessage, SendMessage};
 use async_trait::async_trait;
 use reqwest::Client;
 use serde::Deserialize;
@@ -117,7 +117,7 @@ impl Channel for MatrixChannel {
         "matrix"
     }
 
-    async fn send(&self, message: &str, _target: &str) -> anyhow::Result<()> {
+    async fn send(&self, message: &SendMessage) -> anyhow::Result<()> {
         let txn_id = format!("zc_{}", chrono::Utc::now().timestamp_millis());
         let url = format!(
             "{}/_matrix/client/v3/rooms/{}/send/m.room.message/{}",
@@ -126,7 +126,7 @@ impl Channel for MatrixChannel {
 
         let body = serde_json::json!({
             "msgtype": "m.text",
-            "body": message
+            "body": message.content
         });
 
         let resp = self