perf(zip): eliminate N+1 queries with ltree bulk subtree fetch

Replace BFS traversal that issued 2 SQL queries per folder (list_files +
list_folders) with 2 total queries using PostgreSQL ltree <@ operator:

1. list_subtree_folders: single GiST-indexed scan for all folders
2. list_files_in_subtree: single GiST-indexed join for all files

Files are grouped by folder_id in a HashMap, then iterated in directory
order (folders pre-sorted by path from SQL).

Changes across 4 architecture layers:
- Domain: FolderRepository::list_subtree_folders (default impl)
- Application ports: FolderUseCase, FileRetrievalUseCase, FileReadPort
- Application services: FolderService, FileRetrievalService passthroughs
- Infrastructure: PG implementations + ZipService rewrite

Query count: O(N) → O(1). Latency for 100-folder tree: ~200 round-trips → 3.

src/application/services/folder_service.rs

@@ -182,6 +182,14 @@ impl FolderUseCase for FolderService {
         Ok(FolderDto::from(folder))
     }
 
+    async fn list_subtree_folders(
+        &self,
+        folder_id: &str,
+    ) -> Result<Vec<FolderDto>, DomainError> {
+        let folders = self.folder_storage.list_subtree_folders(folder_id).await?;
+        Ok(folders.into_iter().map(FolderDto::from).collect())
+    }
+
     /// Gets a folder by its ID
     async fn get_folder(&self, id: &str) -> Result<FolderDto, DomainError> {
         let folder = self.folder_storage.get_folder(id).await.map_err(|e| {