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/application/src/watchlist/tests/is_on.rs

@@ -0,0 +1,56 @@
+use std::sync::Arc;
+
+use domain::models::WatchlistEntry;
+use domain::ports::WatchlistRepository;
+use domain::testing::InMemoryWatchlistRepository;
+use domain::value_objects::{MovieId, UserId};
+use uuid::Uuid;
+
+use crate::test_helpers::TestContextBuilder;
+use crate::watchlist::{is_on, queries::IsOnWatchlistQuery};
+
+#[tokio::test]
+async fn returns_true_when_present() {
+    let watchlist = InMemoryWatchlistRepository::new();
+    let uid = Uuid::new_v4();
+    let mid = Uuid::new_v4();
+    watchlist
+        .add(&WatchlistEntry::new(
+            UserId::from_uuid(uid),
+            MovieId::from_uuid(mid),
+        ))
+        .await
+        .unwrap();
+
+    let ctx = TestContextBuilder::new()
+        .with_watchlist(Arc::clone(&watchlist) as _)
+        .build();
+
+    let result = is_on::execute(
+        &ctx,
+        IsOnWatchlistQuery {
+            user_id: uid,
+            movie_id: mid,
+        },
+    )
+    .await
+    .unwrap();
+
+    assert!(result);
+}
+
+#[tokio::test]
+async fn returns_false_when_absent() {
+    let ctx = TestContextBuilder::new().build();
+    let result = is_on::execute(
+        &ctx,
+        IsOnWatchlistQuery {
+            user_id: Uuid::new_v4(),
+            movie_id: Uuid::new_v4(),
+        },
+    )
+    .await
+    .unwrap();
+
+    assert!(!result);
+}