refactor: group use cases into DDD bounded contexts

Flat use_cases/ (44 files) + monolithic commands.rs/queries.rs
split into diary/, movies/, watchlist/, import/, auth/, users/,
integrations/, search/, person/, federation/ — each with own
commands.rs, queries.rs, and use case modules.

Inline tests extracted to sibling tests/ dirs.

crates/application/src/diary/tests/delete_review.rs

@@ -0,0 +1,104 @@
+use std::sync::Arc;
+
+use chrono::Utc;
+
+use domain::{
+    models::{Movie, Review},
+    ports::{MovieRepository, ReviewRepository},
+    testing::{
+        FakeDiaryRepository, InMemoryMovieRepository, InMemoryReviewRepository, NoopEventPublisher,
+    },
+    value_objects::{MovieId, MovieTitle, Rating, ReleaseYear, UserId},
+};
+
+use crate::{
+    diary::commands::DeleteReviewCommand, diary::delete_review, test_helpers::TestContextBuilder,
+};
+
+fn make_movie() -> Movie {
+    Movie::new(
+        None,
+        MovieTitle::new("Terminator".into()).unwrap(),
+        ReleaseYear::new(1984).unwrap(),
+        None,
+        None,
+    )
+}
+
+fn make_review(movie_id: MovieId, user_id: UserId) -> Review {
+    Review::new(
+        movie_id,
+        user_id,
+        Rating::new(4).unwrap(),
+        None,
+        Utc::now().naive_utc(),
+    )
+    .unwrap()
+}
+
+#[tokio::test]
+async fn test_delete_review_removes_it() {
+    let movies = InMemoryMovieRepository::new();
+    let reviews = InMemoryReviewRepository::new();
+    let diary = FakeDiaryRepository::new();
+    let events = NoopEventPublisher::new();
+
+    let movie = make_movie();
+    let user_id = UserId::from_uuid(uuid::Uuid::new_v4());
+    let review = make_review(movie.id().clone(), user_id.clone());
+
+    movies.upsert_movie(&movie).await.unwrap();
+    reviews.save_review(&review).await.unwrap();
+    diary.seed_history(movie.clone(), vec![]);
+
+    let ctx = TestContextBuilder::new()
+        .with_movies(Arc::clone(&movies) as _)
+        .with_reviews(Arc::clone(&reviews) as _)
+        .with_diary(Arc::clone(&diary) as _)
+        .with_event_publisher(Arc::clone(&events) as _)
+        .build();
+
+    delete_review::execute(
+        &ctx,
+        DeleteReviewCommand {
+            review_id: review.id().value(),
+            requesting_user_id: user_id.value(),
+        },
+    )
+    .await
+    .unwrap();
+
+    assert_eq!(reviews.count(), 0, "review should be deleted");
+    assert!(
+        movies.get_movie_by_id(movie.id()).await.unwrap().is_none(),
+        "movie should be deleted when no reviews remain"
+    );
+}
+
+#[tokio::test]
+async fn test_delete_review_wrong_user_is_unauthorized() {
+    let reviews = InMemoryReviewRepository::new();
+
+    let movie_id = MovieId::from_uuid(uuid::Uuid::new_v4());
+    let owner_id = UserId::from_uuid(uuid::Uuid::new_v4());
+    let other_id = uuid::Uuid::new_v4();
+    let review = make_review(movie_id, owner_id);
+
+    reviews.save_review(&review).await.unwrap();
+
+    let ctx = TestContextBuilder::new()
+        .with_reviews(Arc::clone(&reviews) as _)
+        .build();
+
+    let result = delete_review::execute(
+        &ctx,
+        DeleteReviewCommand {
+            review_id: review.id().value(),
+            requesting_user_id: other_id,
+        },
+    )
+    .await;
+
+    assert!(result.is_err(), "wrong user should not be able to delete");
+    assert_eq!(reviews.count(), 1, "review should still exist");
+}