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/user.rs

@@ -0,0 +1,163 @@
+use crate::value_objects::{Email, PasswordHash, UserId, Username};
+
+#[derive(Clone, Debug, Default)]
+pub enum UserRole {
+    #[default]
+    Standard,
+    Admin,
+}
+
+impl UserRole {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::Standard => "standard",
+            Self::Admin => "admin",
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ProfileField {
+    pub name: String,
+    pub value: String,
+}
+
+#[derive(Clone, Debug, Default)]
+pub struct UserProfile {
+    pub display_name: Option<String>,
+    pub bio: Option<String>,
+    pub avatar_path: Option<String>,
+    pub banner_path: Option<String>,
+    pub also_known_as: Option<String>,
+    pub profile_fields: Vec<ProfileField>,
+}
+
+#[derive(Clone, Debug)]
+pub struct User {
+    id: UserId,
+    email: Email,
+    username: Username,
+    password_hash: PasswordHash,
+    role: UserRole,
+    profile: UserProfile,
+}
+
+impl User {
+    pub fn new(
+        email: Email,
+        username: Username,
+        password_hash: PasswordHash,
+        role: UserRole,
+    ) -> Self {
+        Self {
+            id: UserId::generate(),
+            email,
+            username,
+            password_hash,
+            role,
+            profile: UserProfile::default(),
+        }
+    }
+
+    pub fn from_persistence(
+        id: UserId,
+        email: Email,
+        username: Username,
+        password_hash: PasswordHash,
+        role: UserRole,
+        profile: UserProfile,
+    ) -> Self {
+        Self {
+            id,
+            email,
+            username,
+            password_hash,
+            role,
+            profile,
+        }
+    }
+
+    pub fn update_password(&mut self, new_hash: PasswordHash) {
+        self.password_hash = new_hash;
+    }
+
+    pub fn update_profile(&mut self, profile: UserProfile) {
+        self.profile = profile;
+    }
+
+    pub fn email(&self) -> &Email {
+        &self.email
+    }
+    pub fn username(&self) -> &Username {
+        &self.username
+    }
+    pub fn id(&self) -> &UserId {
+        &self.id
+    }
+    pub fn password_hash(&self) -> &PasswordHash {
+        &self.password_hash
+    }
+    pub fn role(&self) -> &UserRole {
+        &self.role
+    }
+    pub fn display_name(&self) -> Option<&str> {
+        self.profile.display_name.as_deref()
+    }
+    pub fn bio(&self) -> Option<&str> {
+        self.profile.bio.as_deref()
+    }
+    pub fn avatar_path(&self) -> Option<&str> {
+        self.profile.avatar_path.as_deref()
+    }
+    pub fn banner_path(&self) -> Option<&str> {
+        self.profile.banner_path.as_deref()
+    }
+    pub fn also_known_as(&self) -> Option<&str> {
+        self.profile.also_known_as.as_deref()
+    }
+    pub fn profile_fields(&self) -> &[ProfileField] {
+        &self.profile.profile_fields
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct UserSummary {
+    pub user_id: UserId,
+    email: Email,
+    username: Username,
+    display_name: Option<String>,
+    pub total_movies: i64,
+    pub avg_rating: Option<f64>,
+    pub avatar_path: Option<String>,
+}
+
+impl UserSummary {
+    pub fn new(
+        user_id: UserId,
+        email: Email,
+        username: Username,
+        display_name: Option<String>,
+        total_movies: i64,
+        avg_rating: Option<f64>,
+        avatar_path: Option<String>,
+    ) -> Self {
+        Self {
+            user_id,
+            email,
+            username,
+            display_name,
+            total_movies,
+            avg_rating,
+            avatar_path,
+        }
+    }
+    pub fn email(&self) -> &str {
+        self.email.value()
+    }
+    pub fn username(&self) -> &str {
+        self.username.value()
+    }
+    pub fn display_name(&self) -> Option<&str> {
+        self.display_name.as_deref()
+    }
+}