perf: replace recursive CTE with ltree <@ in delete_folder

Both delete_folder and delete_folder_permanently used WITH RECURSIVE
to find descendant folders before deleting their files.  This scans
the parent_id chain row-by-row (O(depth × N rows)).

Replace with:
  DELETE FROM storage.files
   WHERE folder_id IN (
       SELECT id FROM storage.folders
        WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid)
   )

The GiST index on lpath resolves the entire subtree in O(log N),
matching the pattern already used by list_subtree_folders,
stream_files_in_subtree, and search_files_in_subtree.

src/infrastructure/repositories/pg/folder_db_repository.rs

@@ -465,17 +465,15 @@ impl FolderRepository for FolderDbRepository {
     }
 
     async fn delete_folder(&self, id: &str) -> Result<(), DomainError> {
-        // First, delete all files in this folder and descendant folders
+        // Delete all files whose folder is anywhere in the subtree.
-        // to avoid constraint violations from ON DELETE SET NULL
+        // Uses the GiST-indexed ltree `<@` operator — O(log N) vs the
+        // O(depth × N) recursive CTE it replaces.
         sqlx::query(
-            r#"
+            "DELETE FROM storage.files \
-            WITH RECURSIVE descendants AS (
+              WHERE folder_id IN ( \
-                SELECT id FROM storage.folders WHERE id = $1::uuid
+                  SELECT id FROM storage.folders \
-                UNION ALL
+                   WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid) \
-                SELECT f.id FROM storage.folders f JOIN descendants d ON f.parent_id = d.id
+              )",
-            )
-            DELETE FROM storage.files WHERE folder_id IN (SELECT id FROM descendants)
-            "#,
         )
         .bind(id)
         .execute(self.pool())
@@ -594,16 +592,14 @@ impl FolderRepository for FolderDbRepository {
     }
 
     async fn delete_folder_permanently(&self, folder_id: &str) -> Result<(), DomainError> {
-        // First, delete all files in this folder and descendant folders
+        // Delete all files whose folder is anywhere in the subtree
+        // (GiST ltree index, same pattern as delete_folder).
         sqlx::query(
-            r#"
+            "DELETE FROM storage.files \
-            WITH RECURSIVE descendants AS (
+              WHERE folder_id IN ( \
-                SELECT id FROM storage.folders WHERE id = $1::uuid
+                  SELECT id FROM storage.folders \
-                UNION ALL
+                   WHERE lpath <@ (SELECT lpath FROM storage.folders WHERE id = $1::uuid) \
-                SELECT f.id FROM storage.folders f JOIN descendants d ON f.parent_id = d.id
+              )",
-            )
-            DELETE FROM storage.files WHERE folder_id IN (SELECT id FROM descendants)
-            "#,
         )
         .bind(folder_id)
         .execute(self.pool())