build(deps): bump langchain from 0.2.14 to 0.2.15 in /backend

Bumps [langchain](https://github.com/langchain-ai/langchain) from 0.2.14 to 0.2.15.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](https://github.com/langchain-ai/langchain/compare/langchain==0.2.14...langchain==0.2.15)

---
updated-dependencies:
- dependency-name: langchain
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

backend/requirements.txt

@@ -34,7 +34,7 @@ anthropic
 google-generativeai==0.7.2
 tiktoken
 
-langchain==0.2.14
+langchain==0.2.15
 langchain-community==0.2.12
 langchain-chroma==0.1.2