refactor: rename soft-delete marker from .deleted to .closed

Internal rename: DELETE_MARKER -> CLOSE_MARKER, DeleteMarker ->
CloseMarker, is_deleted -> is_closed. The batch UUID is dropped;
undo now groups cases by their exact closed_at timestamp, which
bulk-close writes identically across its selection while per-case
close writes a unique stamp.

Behavior unchanged. No migration (project is pre-production);
legacy .deleted files in tmpdata were removed manually.

server/tests/oneliners_api_test.rs

@@ -75,7 +75,7 @@ async fn write_state(case_dir: &Path, state: &OnelinerState) {
 }
 
 async fn mark_deleted(case_dir: &Path) {
-    tokio::fs::write(case_dir.join(".deleted"), "{}")
+    tokio::fs::write(case_dir.join(".closed"), "{}")
         .await
         .unwrap();
 }