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/auth/mod.rs

@@ -0,0 +1,5 @@
+pub mod commands;
+pub mod login;
+pub mod queries;
+pub mod register;
+pub mod register_and_login;