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/search.rs

@@ -1,7 +1,8 @@
 use crate::models::{
-    Movie, MovieId, MovieProfile, Person, PersonId,
+    Movie, MovieProfile, Person, PersonId,
     collections::{PageParams, Paginated},
 };
+use crate::value_objects::MovieId;
 
 #[derive(Clone, Debug, Default)]
 pub struct SearchQuery {