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>

---------

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

backend/src/agents/checkpointer/async_provider.py

@@ -0,0 +1,107 @@
+"""Async checkpointer factory.
+
+Provides an **async context manager** for long-running async servers that need
+proper resource cleanup.
+
+Supported backends: memory, sqlite, postgres.
+
+Usage (e.g. FastAPI lifespan)::
+
+    from src.agents.checkpointer.async_provider import make_checkpointer
+
+    async with make_checkpointer() as checkpointer:
+        app.state.checkpointer = checkpointer  # None if not configured
+
+For sync usage see :mod:`src.agents.checkpointer.provider`.
+"""
+
+from __future__ import annotations
+
+import contextlib
+import logging
+from collections.abc import AsyncIterator
+
+from langgraph.types import Checkpointer
+
+from src.agents.checkpointer.provider import (
+    POSTGRES_CONN_REQUIRED,
+    POSTGRES_INSTALL,
+    SQLITE_INSTALL,
+    _resolve_sqlite_conn_str,
+)
+from src.config.app_config import get_app_config
+
+logger = logging.getLogger(__name__)
+
+# ---------------------------------------------------------------------------
+# Async factory
+# ---------------------------------------------------------------------------
+
+
+@contextlib.asynccontextmanager
+async def _async_checkpointer(config) -> AsyncIterator[Checkpointer]:
+    """Async context manager that constructs and tears down a checkpointer."""
+    if config.type == "memory":
+        from langgraph.checkpoint.memory import InMemorySaver
+
+        yield InMemorySaver()
+        return
+
+    if config.type == "sqlite":
+        try:
+            from langgraph.checkpoint.sqlite.aio import AsyncSqliteSaver
+        except ImportError as exc:
+            raise ImportError(SQLITE_INSTALL) from exc
+
+        import pathlib
+
+        conn_str = _resolve_sqlite_conn_str(config.connection_string or "store.db")
+        # Only create parent directories for real filesystem paths
+        if conn_str != ":memory:" and not conn_str.startswith("file:"):
+            pathlib.Path(conn_str).parent.mkdir(parents=True, exist_ok=True)
+        async with AsyncSqliteSaver.from_conn_string(conn_str) as saver:
+            await saver.setup()
+            yield saver
+        return
+
+    if config.type == "postgres":
+        try:
+            from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
+        except ImportError as exc:
+            raise ImportError(POSTGRES_INSTALL) from exc
+
+        if not config.connection_string:
+            raise ValueError(POSTGRES_CONN_REQUIRED)
+
+        async with AsyncPostgresSaver.from_conn_string(config.connection_string) as saver:
+            await saver.setup()
+            yield saver
+        return
+
+    raise ValueError(f"Unknown checkpointer type: {config.type!r}")
+
+
+# ---------------------------------------------------------------------------
+# Public async context manager
+# ---------------------------------------------------------------------------
+
+
+@contextlib.asynccontextmanager
+async def make_checkpointer() -> AsyncIterator[Checkpointer | None]:
+    """Async context manager that yields a checkpointer for the caller's lifetime.
+    Resources are opened on enter and closed on exit — no global state::
+
+        async with make_checkpointer() as checkpointer:
+            app.state.checkpointer = checkpointer
+
+    Yields ``None`` when no checkpointer is configured in *config.yaml*.
+    """
+
+    config = get_app_config()
+
+    if config.checkpointer is None:
+        yield None
+        return
+
+    async with _async_checkpointer(config.checkpointer) as saver:
+        yield saver