feat: enhance Discord embed with user avatar and author details

crates/jetstream/src/repo.rs

···
                                 did: did.to_string(),
                                 display_name: users[0].display_name.clone(),
                                 handle: users[0].handle.clone(),
+                                avatar_url: users[0].avatar.clone(),
                             },
                             track: discord::model::Track {
                                 title: scrobble_record.title.clone(),

crates/jetstream/src/webhook/discord/mod.rs

···
     desc.push_str(&format!("\non *{}*", esc(&s.track.album)));
 
     DiscordEmbed {
-        title: s.user.display_name.clone(),
+        title: String::new(),
         url,
         description: Some(desc),
         timestamp: Some(s.played_at.clone()),
···
         footer: Some(DiscordFooter {
             text: format!("Rocksky • {}", s.user.handle.clone()),
         }),
+        author: DiscordAuthor {
+            name: s.user.display_name.clone(),
+            url: format!("https://rocksky.app/profile/{}", s.user.handle),
+            icon_url: s.user.avatar_url.clone(),
+        },
     }
 }
 

crates/jetstream/src/webhook/discord/model.rs

···
     pub did: String,
     pub display_name: String,
     pub handle: String,
+    pub avatar_url: String,
 }
 
 #[derive(Debug, Deserialize, Serialize, Clone)]
···
     pub thumbnail: Option<DiscordThumb>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub footer: Option<DiscordFooter>,
+    pub author: DiscordAuthor,
+}
+
+#[derive(Debug, Serialize)]
+pub struct DiscordAuthor {
+    pub name: String,
+    pub url: String,
+    pub icon_url: String,
 }
 
 #[derive(Debug, Serialize)]