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/src/analyze/auto_trigger.rs

@@ -175,7 +175,7 @@ pub async fn try_enqueue_all_for_user(
         if !case_path.is_dir() {
             continue;
         }
-        if crate::paths::is_deleted(&case_path).await {
+        if crate::paths::is_closed(&case_path).await {
             continue;
         }
         try_enqueue(&case_path, tx, config, events_tx).await;