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/slack.rs

@@ -1,4 +1,4 @@
-use super::traits::{Channel, ChannelMessage};
+use super::traits::{Channel, ChannelMessage, SendMessage};
 use async_trait::async_trait;
 
 /// Slack channel — polls conversations.history via Web API
@@ -51,10 +51,10 @@ impl Channel for SlackChannel {
         "slack"
     }
 
-    async fn send(&self, message: &str, channel: &str) -> anyhow::Result<()> {
+    async fn send(&self, message: &SendMessage) -> anyhow::Result<()> {
         let body = serde_json::json!({
-            "channel": channel,
-            "text": message
+            "channel": message.recipient,
+            "text": message.content
         });
 
         let resp = self