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/watchlist/tests/add.rs

@@ -0,0 +1,87 @@
+use std::sync::Arc;
+
+use domain::{
+    models::Movie,
+    ports::MovieRepository,
+    testing::{InMemoryMovieRepository, InMemoryWatchlistRepository},
+    value_objects::{MovieTitle, ReleaseYear},
+};
+
+use crate::{
+    diary::commands::MovieInput, test_helpers::TestContextBuilder, watchlist::add,
+    watchlist::commands::AddToWatchlistCommand,
+};
+
+#[tokio::test]
+async fn test_add_to_watchlist_resolves_and_saves() {
+    let movies = InMemoryMovieRepository::new();
+    let watchlist = InMemoryWatchlistRepository::new();
+
+    let movie = Movie::new(
+        None,
+        MovieTitle::new("The Thing".into()).unwrap(),
+        ReleaseYear::new(1982).unwrap(),
+        None,
+        None,
+    );
+    let movie_uuid = movie.id().value();
+    movies.upsert_movie(&movie).await.unwrap();
+
+    let ctx = TestContextBuilder::new()
+        .with_movies(Arc::clone(&movies) as _)
+        .with_watchlist(Arc::clone(&watchlist) as _)
+        .build();
+
+    let cmd = AddToWatchlistCommand {
+        user_id: uuid::Uuid::new_v4(),
+        input: MovieInput {
+            movie_id: Some(movie_uuid),
+            external_metadata_id: None,
+            manual_title: None,
+            manual_release_year: None,
+            manual_director: None,
+        },
+    };
+
+    add::execute(&ctx, cmd).await.unwrap();
+
+    assert_eq!(watchlist.count(), 1);
+}
+
+#[tokio::test]
+async fn test_add_to_watchlist_already_present_is_idempotent() {
+    let movies = InMemoryMovieRepository::new();
+    let watchlist = InMemoryWatchlistRepository::new();
+
+    let movie = Movie::new(
+        None,
+        MovieTitle::new("RoboCop".into()).unwrap(),
+        ReleaseYear::new(1987).unwrap(),
+        None,
+        None,
+    );
+    let movie_uuid = movie.id().value();
+    let user_id = uuid::Uuid::new_v4();
+    movies.upsert_movie(&movie).await.unwrap();
+
+    let ctx = TestContextBuilder::new()
+        .with_movies(Arc::clone(&movies) as _)
+        .with_watchlist(Arc::clone(&watchlist) as _)
+        .build();
+
+    let make_cmd = || AddToWatchlistCommand {
+        user_id,
+        input: MovieInput {
+            movie_id: Some(movie_uuid),
+            external_metadata_id: None,
+            manual_title: None,
+            manual_release_year: None,
+            manual_director: None,
+        },
+    };
+
+    add::execute(&ctx, make_cmd()).await.unwrap();
+    add::execute(&ctx, make_cmd()).await.unwrap();
+
+    assert_eq!(watchlist.count(), 1, "idempotent add should not duplicate");
+}