API fixes
- /files/search/folder now honors workspace_id, or derives it from the folder.
- search_filters now pass through to Typesense search instead of being dropped.
- Invalid filter ids now return 400 instead of silent no-op.
- Empty term: "" works for filter-only searches by using * internally.
- Date filters no longer hit missing helper code.
- Filtered searches avoid the secondary image-vector collection, which cannot apply these file metadata filters.