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/movies/mod.rs

@@ -0,0 +1,10 @@
+pub mod commands;
+pub mod discovery_indexer;
+pub mod enrich_movie;
+pub mod get_movies;
+pub mod queries;
+pub mod search_cleanup;
+pub mod sync_poster;
+
+pub use discovery_indexer::MovieDiscoveryIndexer;
+pub use search_cleanup::SearchCleanupHandler;