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/application/src/ports.rs

@@ -29,16 +29,6 @@ impl HtmlPageContext {
     }
 }
 
-#[derive(Clone, Debug)]
-pub struct WatchlistDisplayEntry {
-    pub poster_url: Option<String>,
-    pub movie_title: String,
-    pub release_year: u16,
-    pub movie_url: Option<String>,
-    pub added_at: String,
-    pub remove_url: Option<String>,
-}
-
 pub trait RssFeedRenderer: Send + Sync {
     fn render_feed(&self, entries: &[DiaryEntry], title: &str) -> Result<String, String>;
 }