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/watchlist/mod.rs

@@ -1,7 +1,6 @@
 pub mod add;
 pub mod commands;
 pub mod get;
-pub mod get_page;
 pub mod is_on;
 pub mod queries;
 pub mod remove;