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/diary/mod.rs

@@ -0,0 +1,10 @@
+pub mod commands;
+pub mod delete_review;
+pub mod export_diary;
+pub mod get_activity_feed;
+pub mod get_diary;
+pub mod get_movie_social_page;
+pub mod get_review_history;
+pub mod log_review;
+pub mod movie_resolver;
+pub mod queries;