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/enrichment.rs

@@ -0,0 +1,54 @@
+use chrono::{DateTime, Utc};
+
+use crate::value_objects::MovieId;
+
+#[derive(Clone, Debug)]
+pub struct Genre {
+    pub tmdb_id: u32,
+    pub name: String,
+}
+
+#[derive(Clone, Debug)]
+pub struct Keyword {
+    pub tmdb_id: u32,
+    pub name: String,
+}
+
+#[derive(Clone, Debug)]
+pub struct CastMember {
+    pub tmdb_person_id: u64,
+    pub name: String,
+    pub character: String,
+    pub billing_order: u32,
+    pub profile_path: Option<String>,
+}
+
+#[derive(Clone, Debug)]
+pub struct CrewMember {
+    pub tmdb_person_id: u64,
+    pub name: String,
+    pub job: String,
+    pub department: String,
+    pub profile_path: Option<String>,
+}
+
+#[derive(Clone, Debug)]
+pub struct MovieProfile {
+    pub movie_id: MovieId,
+    pub tmdb_id: u64,
+    pub imdb_id: Option<String>,
+    pub overview: Option<String>,
+    pub tagline: Option<String>,
+    pub runtime_minutes: Option<u32>,
+    pub budget_usd: Option<i64>,
+    pub revenue_usd: Option<i64>,
+    pub vote_average: Option<f64>,
+    pub vote_count: Option<u32>,
+    pub original_language: Option<String>,
+    pub collection_name: Option<String>,
+    pub genres: Vec<Genre>,
+    pub keywords: Vec<Keyword>,
+    pub cast: Vec<CastMember>,
+    pub crew: Vec<CrewMember>,
+    pub enriched_at: DateTime<Utc>,
+}