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>

README.md

@@ -58,6 +58,7 @@ DeerFlow has newly integrated the intelligent search and crawling toolset indepe
       - [Sandbox Mode](#sandbox-mode)
       - [MCP Server](#mcp-server)
       - [IM Channels](#im-channels)
+      - [LangSmith Tracing](#langsmith-tracing)
   - [From Deep Research to Super Agent Harness](#from-deep-research-to-super-agent-harness)
   - [Core Features](#core-features)
     - [Skills \& Tools](#skills--tools)
@@ -391,6 +392,21 @@ Once a channel is connected, you can interact with DeerFlow directly from the ch
 
 > Messages without a command prefix are treated as regular chat — DeerFlow creates a thread and responds conversationally.
 
+#### LangSmith Tracing
+
+DeerFlow has built-in [LangSmith](https://smith.langchain.com) integration for observability. When enabled, all LLM calls, agent runs, and tool executions are traced and visible in the LangSmith dashboard.
+
+Add the following to your `.env` file:
+
+```bash
+LANGSMITH_TRACING=true
+LANGSMITH_ENDPOINT=https://api.smith.langchain.com
+LANGSMITH_API_KEY=lsv2_pt_xxxxxxxxxxxxxxxx
+LANGSMITH_PROJECT=xxx
+```
+
+For Docker deployments, tracing is disabled by default. Set `LANGSMITH_TRACING=true` and `LANGSMITH_API_KEY` in your `.env` to enable it.
+
 ## From Deep Research to Super Agent Harness
 
 DeerFlow started as a Deep Research framework — and the community ran with it. Since launch, developers have pushed it far beyond research: building data pipelines, generating slide decks, spinning up dashboards, automating content workflows. Things we never anticipated.