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/ports/inbound.rs

@@ -71,6 +71,18 @@ pub trait FolderUseCase: Send + Sync + 'static {
         user_id: &str,
         name: String,
     ) -> Result<FolderDto, DomainError>;
+
+    /// Lists every folder in a subtree rooted at `folder_id` (inclusive),
+    /// ordered by path.  Uses ltree `<@` — single GiST-indexed query.
+    ///
+    /// Default: returns an empty vec (stubs / mocks).
+    async fn list_subtree_folders(
+        &self,
+        folder_id: &str,
+    ) -> Result<Vec<FolderDto>, DomainError> {
+        let _ = folder_id;
+        Ok(Vec::new())
+    }
 }
 
 /**