feat(apps): 新增 DeerFlow 应用脚手架，并修复 store 的 database 回退

apps/：新增「基于 DeerFlow 的应用」目录，与 backend/、frontend/ 平级，
位于「app 消费 deerflow、不反向依赖」边界的正确侧。含两种集成示例：
  - examples/http-chat   —— HTTP Gateway (REST+SSE)，含登录/CSRF/建线程/流式对话
  - examples/embedded-chat —— 进程内直接调 DeerFlowClient
README 说明边界规则、两种模式、鉴权流程及新建应用约定。

runtime/store：修复 make_store 缺失的 database 段回退。原先 store 工厂只读
legacy 的 checkpointer 段，导致仅配 database:postgres 时，checkpointer 走了
Postgres、但 store 仍回退 InMemoryStore（并打出误导性的「线程列表会丢失」告警，
实际线程在 threads_meta 表里、本就持久）。现对齐 checkpointer 工厂的优先级：
checkpointer 段 → database 段 → InMemoryStore；postgres 分支同样剥掉 +asyncpg
方言前缀，使一个 DATABASE_URL 同时满足 SQLAlchemy 与 LangGraph 的 psycopg store。
告警文案也修正为「跨线程 store 数据会丢失」。

tests：新增 test_store_provider.py（3 例，TDD）覆盖 database→postgres 回退、
无配置时的内存回退、以及 checkpointer 段优先级。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_store_provider.py

@@ -0,0 +1,89 @@
+"""Tests for the async Store factory's backend selection.
+
+Mirrors the checkpointer factory: when no legacy ``checkpointer`` section
+is configured but a unified ``database`` section is, the store must use
+that database backend instead of silently falling back to InMemoryStore.
+"""
+
+import sys
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from deerflow.config.database_config import DatabaseConfig
+from deerflow.runtime.store.async_provider import make_store
+
+
+@pytest.fixture
+def anyio_backend() -> str:
+    return "asyncio"
+
+
+class TestStoreDatabaseFallback:
+    @pytest.mark.anyio
+    async def test_postgres_store_from_database_when_no_checkpointer_section(self):
+        """make_store uses AsyncPostgresStore from the database section when
+        no legacy checkpointer section is present. The +asyncpg dialect prefix
+        is stripped so the same DATABASE_URL satisfies both SQLAlchemy and
+        LangGraph's psycopg-based store."""
+        mock_config = MagicMock()
+        mock_config.checkpointer = None
+        mock_config.database = DatabaseConfig(
+            backend="postgres",
+            postgres_url="postgresql+asyncpg://postgres:pw@localhost:5432/deerflow",
+        )
+
+        mock_store = AsyncMock()
+        mock_cm = AsyncMock()
+        mock_cm.__aenter__.return_value = mock_store
+        mock_cm.__aexit__.return_value = False
+
+        mock_store_cls = MagicMock()
+        mock_store_cls.from_conn_string.return_value = mock_cm
+
+        mock_module = MagicMock()
+        mock_module.AsyncPostgresStore = mock_store_cls
+
+        with (
+            patch("deerflow.runtime.store.async_provider.get_app_config", return_value=mock_config),
+            patch.dict(sys.modules, {"langgraph.store.postgres.aio": mock_module}),
+        ):
+            async with make_store() as store:
+                assert store is mock_store
+
+        # dialect prefix stripped to libpq conninfo
+        mock_store_cls.from_conn_string.assert_called_once_with("postgresql://postgres:pw@localhost:5432/deerflow")
+        mock_store.setup.assert_awaited_once()
+
+    @pytest.mark.anyio
+    async def test_memory_when_no_checkpointer_and_no_database(self):
+        """With neither a checkpointer section nor a non-memory database,
+        the store falls back to InMemoryStore."""
+        from langgraph.store.memory import InMemoryStore
+
+        mock_config = MagicMock()
+        mock_config.checkpointer = None
+        mock_config.database = DatabaseConfig(backend="memory")
+
+        with patch("deerflow.runtime.store.async_provider.get_app_config", return_value=mock_config):
+            async with make_store() as store:
+                assert isinstance(store, InMemoryStore)
+
+    @pytest.mark.anyio
+    async def test_checkpointer_section_takes_precedence_over_database(self):
+        """A legacy checkpointer section still wins over the database section."""
+        from deerflow.config.checkpointer_config import CheckpointerConfig
+
+        mock_config = MagicMock()
+        mock_config.checkpointer = CheckpointerConfig(type="memory")
+        mock_config.database = DatabaseConfig(
+            backend="postgres",
+            postgres_url="postgresql+asyncpg://postgres:pw@localhost:5432/deerflow",
+        )
+
+        from langgraph.store.memory import InMemoryStore
+
+        with patch("deerflow.runtime.store.async_provider.get_app_config", return_value=mock_config):
+            async with make_store() as store:
+                # checkpointer.type == memory → InMemoryStore, database ignored
+                assert isinstance(store, InMemoryStore)