perf(grants,favorites): batch resource resolution to kill N+1 and pool fan-out

Three list endpoints resolved each resource with one query per id:

- GET /api/grants/incoming and /api/grants/outgoing used
  join_all(ids.map(get_file)) + join_all(ids.map(get_folder)), so a single
  page (limit ≤ 200) could demand ~200 concurrent connections from the
  20-connection primary pool, causing acquire-timeouts and head-of-line
  blocking under load.
- The NextCloud favorites REPORT (oc:filter-files) fetched get_file/
  get_folder once per favorite — up to N serial round-trips per sync.

Add by-ids batch reads that mirror the existing get_file/get_folder column
mapping and NOT is_trashed filter:

- FileBlobReadRepository::get_files_by_ids / FolderDbRepository::get_folders_by_ids
  (one SELECT ... WHERE id = ANY($1)), exposed as FileRetrievalService::
  get_files_by_ids / FolderService::get_folders_by_ids returning DTOs.
- Both grant handlers and the favorites REPORT now issue two batch queries
  total and look results up by id, preserving original order. Missing ids
  (stale grants whose resource was deleted, or trashed/removed favorites)
  drop out exactly as before. No auth-semantics change: these paths already
  resolved ids vetted by the authorization engine / favorites table.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TAzLEQDaLak3dnrEN3YT35

src/application/services/folder_service.rs

@@ -29,6 +29,17 @@ impl FolderService {
         }
     }
 
+    /// Batch counterpart of `get_folder`: resolve many folder ids in ONE
+    /// query instead of one per id. Like `get_folder` it performs no
+    /// per-folder authorization — both current callers (ACL grant listing,
+    /// NextCloud favorites REPORT) resolve ids already vetted by the
+    /// authorization engine or the favorites table. Missing or trashed ids
+    /// are absent from the result; callers re-associate by `id`.
+    pub async fn get_folders_by_ids(&self, ids: &[String]) -> Result<Vec<FolderDto>, DomainError> {
+        let folders = self.folder_storage.get_folders_by_ids(ids).await?;
+        Ok(folders.into_iter().map(FolderDto::from).collect())
+    }
+
     /// Helper: parse a folder id string into a `Resource::Folder`. Returns
     /// `DomainError::not_found` on parse error (anti-enumeration — the same
     /// error as "folder does not exist").