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/integrations/cleanup.rs

@@ -0,0 +1,12 @@
+use chrono::Duration;
+use domain::errors::DomainError;
+
+use crate::context::AppContext;
+
+pub async fn execute(ctx: &AppContext) -> Result<u64, DomainError> {
+    let cutoff = chrono::Utc::now().naive_utc() - Duration::days(30);
+    ctx.repos
+        .watch_event
+        .delete_non_pending_older_than(cutoff)
+        .await
+}