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/review.rs

@@ -0,0 +1,158 @@
+use chrono::{NaiveDateTime, Utc};
+
+use crate::{
+    errors::DomainError,
+    value_objects::{Comment, MovieId, Rating, ReviewId, UserId},
+};
+
+use super::movie::Movie;
+
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub enum ReviewSource {
+    #[default]
+    Local,
+    Remote {
+        actor_url: String,
+    },
+}
+
+pub struct PersistedReview {
+    pub id: ReviewId,
+    pub movie_id: MovieId,
+    pub user_id: UserId,
+    pub rating: Rating,
+    pub comment: Option<Comment>,
+    pub watched_at: NaiveDateTime,
+    pub created_at: NaiveDateTime,
+    pub source: ReviewSource,
+}
+
+#[derive(Clone, Debug)]
+pub struct Review {
+    id: ReviewId,
+    movie_id: MovieId,
+    user_id: UserId,
+    rating: Rating,
+    comment: Option<Comment>,
+    watched_at: NaiveDateTime,
+    created_at: NaiveDateTime,
+    source: ReviewSource,
+}
+
+impl Review {
+    pub fn new(
+        movie_id: MovieId,
+        user_id: UserId,
+        rating: Rating,
+        comment: Option<Comment>,
+        watched_at: NaiveDateTime,
+    ) -> Result<Self, DomainError> {
+        Ok(Self {
+            id: ReviewId::generate(),
+            movie_id,
+            user_id,
+            rating,
+            comment,
+            watched_at,
+            created_at: Utc::now().naive_utc(),
+            source: ReviewSource::Local,
+        })
+    }
+
+    pub fn from_persistence(row: PersistedReview) -> Self {
+        Self {
+            id: row.id,
+            movie_id: row.movie_id,
+            user_id: row.user_id,
+            rating: row.rating,
+            comment: row.comment,
+            watched_at: row.watched_at,
+            created_at: row.created_at,
+            source: row.source,
+        }
+    }
+
+    pub fn id(&self) -> &ReviewId {
+        &self.id
+    }
+    pub fn movie_id(&self) -> &MovieId {
+        &self.movie_id
+    }
+    pub fn user_id(&self) -> &UserId {
+        &self.user_id
+    }
+    pub fn rating(&self) -> &Rating {
+        &self.rating
+    }
+    pub fn comment(&self) -> Option<&Comment> {
+        self.comment.as_ref()
+    }
+    pub fn watched_at(&self) -> &NaiveDateTime {
+        &self.watched_at
+    }
+    pub fn created_at(&self) -> &NaiveDateTime {
+        &self.created_at
+    }
+    pub fn source(&self) -> &ReviewSource {
+        &self.source
+    }
+    /// Returns [star1_filled, star2_filled, ..., star5_filled]
+    pub fn stars(&self) -> [bool; 5] {
+        let r = self.rating.value();
+        [r >= 1, r >= 2, r >= 3, r >= 4, r >= 5]
+    }
+
+    pub fn is_remote(&self) -> bool {
+        matches!(self.source, ReviewSource::Remote { .. })
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct DiaryEntry {
+    movie: Movie,
+    review: Review,
+}
+
+impl DiaryEntry {
+    pub fn new(movie: Movie, review: Review) -> Self {
+        Self { movie, review }
+    }
+
+    pub fn movie(&self) -> &Movie {
+        &self.movie
+    }
+    pub fn review(&self) -> &Review {
+        &self.review
+    }
+}
+
+#[derive(Clone, Debug, Default)]
+pub struct DiaryFilter {
+    pub sort_by: super::SortDirection,
+    pub page: crate::models::collections::PageParams,
+    pub movie_id: Option<MovieId>,
+    pub user_id: Option<UserId>,
+    pub search: Option<String>,
+}
+
+#[derive(Clone, Debug)]
+pub struct ReviewHistory {
+    movie: Movie,
+    viewings: Vec<Review>,
+}
+
+impl ReviewHistory {
+    pub fn new(movie: Movie, viewings: Vec<Review>) -> Self {
+        Self { movie, viewings }
+    }
+
+    pub fn movie(&self) -> &Movie {
+        &self.movie
+    }
+    pub fn viewings(&self) -> &[Review] {
+        &self.viewings
+    }
+    pub fn sort_by_date(&mut self) {
+        self.viewings.sort_by_key(|r| *r.watched_at());
+    }
+}