add 400+ unit tests for domain and application layers

Extract ReviewLogger trait to decouple import/integrations
from diary::log_review (cross-module coupling smell).

Add in-memory fakes for all repository ports, enabling
isolated testing of every use case module without a database.

Coverage: domain+application 22% → 80%, 427 tests.

crates/domain/src/services/review_history.rs

@@ -51,3 +51,83 @@ impl ReviewHistoryAnalyzer {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::{Movie, Review, ReviewHistory};
+    use crate::value_objects::{MovieId, MovieTitle, Rating, ReleaseYear, UserId};
+    use chrono::{NaiveDate, NaiveDateTime, NaiveTime};
+
+    fn make_movie() -> Movie {
+        Movie::new(
+            None,
+            MovieTitle::new("Test".into()).unwrap(),
+            ReleaseYear::new(2024).unwrap(),
+            None,
+            None,
+        )
+    }
+
+    fn dt(year: i32, month: u32, day: u32) -> NaiveDateTime {
+        NaiveDateTime::new(
+            NaiveDate::from_ymd_opt(year, month, day).unwrap(),
+            NaiveTime::from_hms_opt(12, 0, 0).unwrap(),
+        )
+    }
+
+    fn review_with_rating(movie_id: &MovieId, rating: u8, watched_at: NaiveDateTime) -> Review {
+        let user_id = UserId::generate();
+        Review::new(
+            movie_id.clone(),
+            user_id,
+            Rating::new(rating).unwrap(),
+            None,
+            watched_at,
+        )
+        .unwrap()
+    }
+
+    #[test]
+    fn neutral_when_empty() {
+        let movie = make_movie();
+        let history = ReviewHistory::new(movie, vec![]);
+        let trend = ReviewHistoryAnalyzer::rating_trend(&history).unwrap();
+        assert_eq!(trend, Trend::Neutral);
+    }
+
+    #[test]
+    fn neutral_when_single_review() {
+        let movie = make_movie();
+        let r = review_with_rating(movie.id(), 4, dt(2024, 1, 1));
+        let history = ReviewHistory::new(movie, vec![r]);
+        let trend = ReviewHistoryAnalyzer::rating_trend(&history).unwrap();
+        assert_eq!(trend, Trend::Neutral);
+    }
+
+    #[test]
+    fn improved_when_latest_above_average() {
+        let movie = make_movie();
+        let viewings = vec![
+            review_with_rating(movie.id(), 2, dt(2024, 1, 1)),
+            review_with_rating(movie.id(), 3, dt(2024, 2, 1)),
+            review_with_rating(movie.id(), 5, dt(2024, 3, 1)),
+        ];
+        let history = ReviewHistory::new(movie, viewings);
+        let trend = ReviewHistoryAnalyzer::rating_trend(&history).unwrap();
+        assert_eq!(trend, Trend::Improved);
+    }
+
+    #[test]
+    fn declined_when_latest_below_average() {
+        let movie = make_movie();
+        let viewings = vec![
+            review_with_rating(movie.id(), 5, dt(2024, 1, 1)),
+            review_with_rating(movie.id(), 4, dt(2024, 2, 1)),
+            review_with_rating(movie.id(), 2, dt(2024, 3, 1)),
+        ];
+        let history = ReviewHistory::new(movie, viewings);
+        let trend = ReviewHistoryAnalyzer::rating_trend(&history).unwrap();
+        assert_eq!(trend, Trend::Declined);
+    }
+}