feat(rss): implement RSS feed adapter and integrate with application state

crates/application/src/ports.rs

@@ -3,3 +3,7 @@ use domain::models::{DiaryEntry, collections::Paginated};
 pub trait HtmlRenderer: Send + Sync {
     fn render_diary_page(&self, data: &Paginated<DiaryEntry>) -> Result<String, String>;
 }
+
+pub trait RssFeedRenderer: Send + Sync {
+    fn render_feed(&self, entries: &[DiaryEntry]) -> Result<String, String>;
+}