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/presentation/src/mappers/users.rs

@@ -1,5 +1,6 @@
 use application::users::get_profile::PendingFollowerView;
-use domain::models::UserSummary;
+use chrono::Datelike;
+use domain::models::{DiaryEntry, MonthActivity, UserSummary};
 use domain::ports::RemoteActorInfo;
 use template_askama::{RemoteActorData, RemoteActorDisplay, UserSummaryView};
 
@@ -43,3 +44,49 @@ pub fn pending_follower_data(p: &PendingFollowerView) -> RemoteActorData {
         avatar_url: p.avatar_url.clone(),
     }
 }
+
+pub fn group_by_month(entries: Vec<DiaryEntry>) -> Vec<MonthActivity> {
+    use std::collections::BTreeMap;
+    let mut map: BTreeMap<(i32, u32), Vec<DiaryEntry>> = BTreeMap::new();
+    for entry in entries {
+        let watched_at = entry.review().watched_at();
+        let year = watched_at.year();
+        let month = watched_at.month();
+        map.entry((year, month)).or_default().push(entry);
+    }
+    map.into_iter()
+        .rev()
+        .map(|((year, month), entries)| {
+            let year_month = format!("{:04}-{:02}", year, month);
+            MonthActivity {
+                month_label: format_year_month_long(&year_month),
+                count: entries.len() as i64,
+                entries,
+                year_month,
+            }
+        })
+        .collect()
+}
+
+fn format_year_month_long(ym: &str) -> String {
+    let parts: Vec<&str> = ym.splitn(2, '-').collect();
+    if parts.len() != 2 {
+        return ym.to_string();
+    }
+    let month = match parts[1] {
+        "01" => "January",
+        "02" => "February",
+        "03" => "March",
+        "04" => "April",
+        "05" => "May",
+        "06" => "June",
+        "07" => "July",
+        "08" => "August",
+        "09" => "September",
+        "10" => "October",
+        "11" => "November",
+        "12" => "December",
+        _ => parts[1],
+    };
+    format!("{} {}", month, parts[0])
+}