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/federation/get_remote_watchlist.rs

@@ -0,0 +1,10 @@
+use domain::{errors::DomainError, models::RemoteWatchlistEntry};
+
+use crate::context::AppContext;
+
+pub async fn execute(
+    ctx: &AppContext,
+    uuid: uuid::Uuid,
+) -> Result<Vec<RemoteWatchlistEntry>, DomainError> {
+    ctx.repos.remote_watchlist.get_by_derived_uuid(uuid).await
+}