Add LangSmith tracing setup instructions across the project:
- .env.example: add LANGSMITH_* env vars (commented out)
- README.md + translations (zh/ja/fr/ru): add LangSmith Tracing section
under Advanced with setup steps and env var reference
- backend/README.md: add detailed LangSmith Tracing section with setup,
env var table, how-it-works explanation, and Docker notes
- docker-compose.yaml: update LANGCHAIN_TRACING_V2 to LANGSMITH_TRACING
for naming consistency with the rest of the project
Made-with: Cursor
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>