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/src/config/paths.py

@@ -176,3 +176,15 @@ def get_paths() -> Paths:
     if _paths is None:
         _paths = Paths()
     return _paths
+
+
+def resolve_path(path: str) -> Path:
+    """Resolve *path* to an absolute ``Path``.
+
+    Relative paths are resolved relative to the application base directory.
+    Absolute paths are returned as-is (after normalisation).
+    """
+    p = Path(path)
+    if not p.is_absolute():
+        p = get_paths().base_dir / path
+    return p.resolve()