Bump langchain from 0.3.27 to 1.0.2 in /backend

Bumps [langchain](https://github.com/langchain-ai/langchain) from 0.3.27 to 1.0.2.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain==0.3.27...langchain==1.0.2)

---
updated-dependencies:
- dependency-name: langchain
  dependency-version: 1.0.2
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Author: dependabot[bot]