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/infrastructure/repositories/pg/folder_db_repository.rs

@@ -109,6 +109,37 @@ impl FolderDbRepository {
         )
         .map_err(|e| DomainError::internal_error("FolderDb", format!("entity: {e}")))
     }
+
+    /// Batch-fetch folders by id — the by-ids counterpart of `get_folder`,
+    /// resolving a page of ACL grants or favorites in ONE query instead of
+    /// one per id. Same `NOT is_trashed` filter and column mapping as
+    /// `get_folder`; missing or trashed ids drop out and callers re-associate
+    /// by id; ordering is not guaranteed.
+    pub async fn get_folders_by_ids(&self, ids: &[String]) -> Result<Vec<Folder>, DomainError> {
+        let uuid_ids: Vec<Uuid> = ids.iter().filter_map(|id| id.parse().ok()).collect();
+        if uuid_ids.is_empty() {
+            return Ok(Vec::new());
+        }
+
+        let rows = sqlx::query_as::<_, FolderRow>(
+            r#"
+            SELECT id::text, name, path, parent_id::text, user_id,
+                   EXTRACT(EPOCH FROM created_at)::bigint,
+                   EXTRACT(EPOCH FROM updated_at)::bigint,
+                   EXTRACT(EPOCH FROM tree_modified_at)::bigint
+              FROM storage.folders
+             WHERE id = ANY($1) AND NOT is_trashed
+            "#,
+        )
+        .bind(&uuid_ids)
+        .fetch_all(self.pool())
+        .await
+        .map_err(|e| DomainError::internal_error("FolderDb", format!("get_folders_by_ids: {e}")))?;
+
+        rows.into_iter()
+            .map(|r| Self::row_to_folder(r.0, r.1, r.2, r.3, Some(r.4), r.5, r.6, r.7))
+            .collect()
+    }
 }
 
 impl FolderRepository for FolderDbRepository {