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/discord.rs

@@ -1,4 +1,4 @@
-use super::traits::{Channel, ChannelMessage};
+use super::traits::{Channel, ChannelMessage, SendMessage};
 use async_trait::async_trait;
 use futures_util::{SinkExt, StreamExt};
 use serde_json::json;
@@ -185,11 +185,15 @@ impl Channel for DiscordChannel {
         "discord"
     }
 
-    async fn send(&self, message: &str, channel_id: &str) -> anyhow::Result<()> {
-        let chunks = split_message_for_discord(message);
+    async fn send(&self, message: &SendMessage) -> anyhow::Result<()> {
+        let chunks = split_message_for_discord(&message.content);
 
         for (i, chunk) in chunks.iter().enumerate() {
-            let url = format!("https://discord.com/api/v10/channels/{channel_id}/messages");
+            let url = format!(
+                "https://discord.com/api/v10/channels/{}/messages",
+                message.recipient
+            );
+
             let body = json!({ "content": chunk });
 
             let resp = self