docs: add LangSmith tracing configuration and documentation (#1414)

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>

.env.example

@@ -24,5 +24,11 @@ INFOQUEST_API_KEY=your-infoquest-api-key
 # SLACK_APP_TOKEN=your-slack-app-token
 # TELEGRAM_BOT_TOKEN=your-telegram-bot-token
 
+# Enable LangSmith to monitor and debug your LLM calls, agent runs, and tool executions.
+# LANGSMITH_TRACING=true
+# LANGSMITH_ENDPOINT=https://api.smith.langchain.com
+# LANGSMITH_API_KEY=your-langsmith-api-key
+# LANGSMITH_PROJECT=your-langsmith-project
+
 # GitHub API Token
 # GITHUB_TOKEN=your-github-token