fix(checkpointer): strip SQLAlchemy dialect prefix before passing to LangGraph

DeerFlow 用同一个 \`postgres_url\` 喂两条路径：
  - SQLAlchemy 引擎（需 \`postgresql+asyncpg://\`）
  - LangGraph AsyncPostgresSaver（psycopg 直连，需 libpq 风格 \`postgresql://\`）

PR2 把 .env / config.example.yaml 默认 URL 改成 \`+asyncpg\` 形态后，LangGraph
saver from_conn_string 会被 psycopg parser 直接抛 ProgrammingError
'missing "=" after "postgresql+asyncpg://..."'。

修：在 async_provider.py 传给 AsyncPostgresSaver 之前用正则剥掉
\`postgresql+\\w+://\` → \`postgresql://\`。一行修复，让同一个 URL 同时满足
SQLAlchemy 和 LangGraph 两条路径。

实测：make dev-daemon 起 gateway 成功，远程 RDS 上 9 张表（DeerFlow 5 +
LangGraph 4）全部 create_all 出来。

Stage 0 PR2 follow-up（不在原 plan task list 内，是 live make dev 触发的）。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/runtime/checkpointer/async_provider.py

@@ -20,6 +20,7 @@ from __future__ import annotations
 import asyncio
 import contextlib
 import logging
+import re
 from collections.abc import AsyncIterator
 
 from langgraph.types import Checkpointer
@@ -114,7 +115,14 @@ async def _async_checkpointer_from_database(db_config) -> AsyncIterator[Checkpoi
         if not db_config.postgres_url:
             raise ValueError("database.postgres_url is required for the postgres backend")
 
-        async with AsyncPostgresSaver.from_conn_string(db_config.postgres_url) as saver:
+        # LangGraph's AsyncPostgresSaver wraps psycopg directly and expects a
+        # libpq-style conninfo (`postgresql://...`). DeerFlow's own SQLAlchemy
+        # engine uses the same `postgres_url` but needs the `+asyncpg` dialect
+        # prefix. Strip the dialect prefix here so the same env var/config
+        # value satisfies both paths.
+        lg_conn_str = re.sub(r"^postgresql\+\w+://", "postgresql://", db_config.postgres_url)
+
+        async with AsyncPostgresSaver.from_conn_string(lg_conn_str) as saver:
             await saver.setup()
             yield saver
         return