Adds a chip-based tag input to the hub grid so users can AND-filter
datasets by tag across HuggingFace, OpenML, and Zenodo.

- Backend: new `tags` query param (repeated) on the search endpoint;
  each source applies it natively (HF `filter=`, OpenML `tag=`,
  Zenodo `keywords:"…"` Lucene clause)
- Frontend: tag chip UI (Enter/comma to add, click X to remove);
  Axios paramsSerializer fixed to emit repeated params instead of
  comma-joining, preventing breakage on tags that contain commas
- OpenML pagination uses limit+1 sentinel pattern to avoid premature
  next_cursor=None when a tag filter reduces page size
- Zenodo escapes `\` and `"` inside Lucene quoted strings
- OpenML multi-tag drop now logged at WARNING level

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>