Support langgraph checkpointer (#1005)

* Add checkpointer configuration to config.example.yaml

- Introduced a new section for checkpointer configuration to enable state persistence for the embedded DeerFlowClient.
- Documented supported types: memory, sqlite, and postgres, along with examples for each.
- Clarified that the LangGraph Server manages its own state persistence separately.

* refactor(checkpointer): streamline checkpointer initialization and logging

* fix(uv.lock): update revision and add new wheel URLs for brotlicffi package

* feat: add langchain-anthropic dependency and update related configurations

* Fix checkpointer lifecycle, docstring, and path resolution bugs from PR #1005 review (#4)

* Initial plan

* Address all review suggestions from PR #1005

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

* Fix resolve_path to always return real Path; move SQLite special-string handling to callers

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

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Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

backend/pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "httpx>=0.28.0",
     "kubernetes>=30.0.0",
     "langchain>=1.2.3",
+    "langchain-anthropic>=1.3.4",
     "langchain-deepseek>=1.0.1",
     "langchain-mcp-adapters>=0.1.0",
     "langchain-openai>=1.1.7",
@@ -32,6 +33,7 @@ dependencies = [
     "ddgs>=9.10.0",
     "duckdb>=1.4.4",
     "langchain-google-genai>=4.2.1",
+    "langgraph-checkpoint-sqlite>=3.0.3",
 ]
 
 [dependency-groups]