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/checkpointer_config.py

@@ -0,0 +1,46 @@
+"""Configuration for LangGraph checkpointer."""
+
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+CheckpointerType = Literal["memory", "sqlite", "postgres"]
+
+
+class CheckpointerConfig(BaseModel):
+    """Configuration for LangGraph state persistence checkpointer."""
+
+    type: CheckpointerType = Field(
+        description="Checkpointer backend type. "
+        "'memory' is in-process only (lost on restart). "
+        "'sqlite' persists to a local file (requires langgraph-checkpoint-sqlite). "
+        "'postgres' persists to PostgreSQL (requires langgraph-checkpoint-postgres)."
+    )
+    connection_string: str | None = Field(
+        default=None,
+        description="Connection string for sqlite (file path) or postgres (DSN). "
+        "Required for sqlite and postgres types. "
+        "For sqlite, use a file path like '.deer-flow/checkpoints.db' or ':memory:' for in-memory. "
+        "For postgres, use a DSN like 'postgresql://user:pass@localhost:5432/db'.",
+    )
+
+
+# Global configuration instance — None means no checkpointer is configured.
+_checkpointer_config: CheckpointerConfig | None = None
+
+
+def get_checkpointer_config() -> CheckpointerConfig | None:
+    """Get the current checkpointer configuration, or None if not configured."""
+    return _checkpointer_config
+
+
+def set_checkpointer_config(config: CheckpointerConfig | None) -> None:
+    """Set the checkpointer configuration."""
+    global _checkpointer_config
+    _checkpointer_config = config
+
+
+def load_checkpointer_config_from_dict(config_dict: dict) -> None:
+    """Load checkpointer configuration from a dictionary."""
+    global _checkpointer_config
+    _checkpointer_config = CheckpointerConfig(**config_dict)