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/queries.rs

@@ -0,0 +1,12 @@
+use uuid::Uuid;
+
+pub struct GetWatchlistQuery {
+    pub user_id: Uuid,
+    pub limit: Option<u32>,
+    pub offset: Option<u32>,
+}
+
+pub struct IsOnWatchlistQuery {
+    pub user_id: Uuid,
+    pub movie_id: Uuid,
+}