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/movie.rs

@@ -0,0 +1,106 @@
+use crate::value_objects::{ExternalMetadataId, MovieId, MovieTitle, PosterPath, ReleaseYear};
+
+#[derive(Clone, Debug, Default)]
+pub struct MovieFilter {
+    pub search: Option<String>,
+    pub genre: Option<String>,
+    pub language: Option<String>,
+}
+
+#[derive(Clone, Debug)]
+pub struct MovieSummary {
+    pub movie: Movie,
+    pub genres: Vec<String>,
+    pub runtime_minutes: Option<u32>,
+    pub original_language: Option<String>,
+    pub overview: Option<String>,
+    pub collection_name: Option<String>,
+}
+
+#[derive(Clone, Debug)]
+pub struct Movie {
+    id: MovieId,
+    external_metadata_id: Option<ExternalMetadataId>,
+    title: MovieTitle,
+    release_year: ReleaseYear,
+    director: Option<String>,
+    poster_path: Option<PosterPath>,
+}
+
+impl Movie {
+    pub fn new(
+        external_metadata_id: Option<ExternalMetadataId>,
+        title: MovieTitle,
+        release_year: ReleaseYear,
+        director: Option<String>,
+        poster_path: Option<PosterPath>,
+    ) -> Self {
+        Self {
+            id: MovieId::generate(),
+            external_metadata_id,
+            title,
+            release_year,
+            director,
+            poster_path,
+        }
+    }
+
+    pub fn from_persistence(
+        id: MovieId,
+        external_metadata_id: Option<ExternalMetadataId>,
+        title: MovieTitle,
+        release_year: ReleaseYear,
+        director: Option<String>,
+        poster_path: Option<PosterPath>,
+    ) -> Self {
+        Self {
+            id,
+            external_metadata_id,
+            title,
+            release_year,
+            director,
+            poster_path,
+        }
+    }
+
+    pub fn update_poster(&mut self, poster_path: PosterPath) {
+        self.poster_path = Some(poster_path);
+    }
+
+    pub fn id(&self) -> &MovieId {
+        &self.id
+    }
+    pub fn external_metadata_id(&self) -> Option<&ExternalMetadataId> {
+        self.external_metadata_id.as_ref()
+    }
+    pub fn title(&self) -> &MovieTitle {
+        &self.title
+    }
+    pub fn release_year(&self) -> &ReleaseYear {
+        &self.release_year
+    }
+    pub fn director(&self) -> Option<&str> {
+        self.director.as_deref()
+    }
+    pub fn poster_path(&self) -> Option<&PosterPath> {
+        self.poster_path.as_ref()
+    }
+}
+
+impl Movie {
+    pub fn is_manual_match(
+        &self,
+        title: &MovieTitle,
+        year: &ReleaseYear,
+        director: Option<&str>,
+    ) -> bool {
+        if self.title != *title || self.release_year != *year {
+            return false;
+        }
+
+        match (self.director(), director) {
+            (Some(existing_dir), Some(new_dir)) => existing_dir.eq_ignore_ascii_case(new_dir),
+            _ => true,
+        }
+    }
+}