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/export_diary.rs

@@ -0,0 +1,15 @@
+use domain::{errors::DomainError, value_objects::UserId};
+
+use crate::{context::AppContext, diary::queries::ExportQuery};
+
+pub async fn execute(ctx: &AppContext, query: ExportQuery) -> Result<Vec<u8>, DomainError> {
+    let entries = ctx
+        .repos
+        .diary
+        .get_user_history(&UserId::from_uuid(query.user_id))
+        .await?;
+    ctx.services
+        .diary_exporter
+        .serialize_entries(&entries, query.format)
+        .await
+}