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/is_on.rs

@@ -0,0 +1,12 @@
+use domain::{
+    errors::DomainError,
+    value_objects::{MovieId, UserId},
+};
+
+use crate::{context::AppContext, watchlist::queries::IsOnWatchlistQuery};
+
+pub async fn execute(ctx: &AppContext, query: IsOnWatchlistQuery) -> Result<bool, DomainError> {
+    let user_id = UserId::from_uuid(query.user_id);
+    let movie_id = MovieId::from_uuid(query.movie_id);
+    ctx.repos.watchlist.contains(&user_id, &movie_id).await
+}