Bump llama-index from 0.9.45 to 0.12.9 in /backend/requirements

Bumps [llama-index](https://github.com/run-llama/llama_index) from 0.9.45 to 0.12.9.
- [Release notes](https://github.com/run-llama/llama_index/releases)
- [Changelog](https://github.com/run-llama/llama_index/blob/main/CHANGELOG.md)
- [Commits](run-llama/llama_index@v0.9.45...v0.12.9)

---
updated-dependencies:
- dependency-name: llama-index
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/requirements/default.txt

@@ -41,7 +41,7 @@ langgraph-sdk==0.1.44
 litellm==1.63.8
 lxml==5.3.0
 lxml_html_clean==0.2.2
-llama-index==0.9.45
+llama-index==0.12.9
 Mako==1.2.4
 msal==1.28.0
 nltk==3.8.1