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/adapters/tmdb-enrichment/src/lib.rs

@@ -1,6 +1,6 @@
 use std::sync::Arc;
 
-use application::{commands::EnrichMovieCommand, use_cases::enrich_movie};
+use application::movies::{commands::EnrichMovieCommand, enrich_movie};
 use async_trait::async_trait;
 use chrono::Utc;
 use domain::{