refactor: split domain models, move presentation logic out of app layer

Split domain/models/mod.rs (630 lines) into focused files:
movie.rs, review.rs, user.rs, stats.rs, enrichment.rs, feed.rs.

Move URL/date formatting from application use cases to
presentation mappers — use cases now return raw domain data.

Delete watchlist/get_page.rs (was pure presentation logic),
replace with presentation/mappers/watchlist.rs.

Document signature conventions in CONTRIBUTING.md.

crates/domain/src/models/feed.rs

@@ -0,0 +1,28 @@
+use super::{movie::Movie, review::{DiaryEntry, Review}};
+
+#[derive(Clone, Debug)]
+pub struct FeedEntry {
+    entry: DiaryEntry,
+    user_email: String,
+}
+
+impl FeedEntry {
+    pub fn new(entry: DiaryEntry, user_email: String) -> Self {
+        Self { entry, user_email }
+    }
+    pub fn movie(&self) -> &Movie {
+        self.entry.movie()
+    }
+    pub fn review(&self) -> &Review {
+        self.entry.review()
+    }
+    pub fn user_email(&self) -> &str {
+        &self.user_email
+    }
+    pub fn user_display_name(&self) -> &str {
+        self.user_email
+            .split('@')
+            .next()
+            .unwrap_or(&self.user_email)
+    }
+}