build(deps): bump elasticsearch from 9.0.1 to 9.1.0 in /backend

Bumps [elasticsearch](https://github.com/elastic/elasticsearch-py) from 9.0.1 to 9.1.0.
- [Release notes](https://github.com/elastic/elasticsearch-py/releases)
- [Commits](https://github.com/elastic/elasticsearch-py/compare/v9.0.1...v9.1.0)

---
updated-dependencies:
- dependency-name: elasticsearch
  dependency-version: 9.1.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/requirements.txt

@@ -56,7 +56,7 @@ pymilvus==2.5.0
 qdrant-client==1.14.3
 opensearch-py==2.8.0
 playwright==1.49.1 # Caution: version must match docker-compose.playwright.yaml
-elasticsearch==9.0.1
+elasticsearch==9.1.0
 pinecone==6.0.2
 oracledb==3.2.0