feat(persistence): add SQLAlchemy 2.0 async ORM scaffold

Introduce a unified database configuration (DatabaseConfig) that
controls both the LangGraph checkpointer and the DeerFlow application
persistence layer from a single `database:` config section.

New modules:
- deerflow.config.database_config — Pydantic config with memory/sqlite/postgres backends
- deerflow.persistence — async engine lifecycle, DeclarativeBase with to_dict mixin, Alembic skeleton
- deerflow.runtime.runs.store — RunStore ABC + MemoryRunStore implementation

Gateway integration initializes/tears down the persistence engine in
the existing langgraph_runtime() context manager. Legacy checkpointer
config is preserved for backward compatibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/runtime/runs/store/base.py

@@ -0,0 +1,59 @@
+"""Abstract interface for run metadata storage.
+
+RunManager depends on this interface. Implementations:
+- MemoryRunStore: in-memory dict (development, tests)
+- Future: RunRepository backed by SQLAlchemy ORM
+
+All methods accept an optional owner_id for user isolation.
+When owner_id is None, no user filtering is applied (single-user mode).
+"""
+
+from __future__ import annotations
+
+import abc
+from typing import Any
+
+
+class RunStore(abc.ABC):
+    @abc.abstractmethod
+    async def put(
+        self,
+        run_id: str,
+        *,
+        thread_id: str,
+        assistant_id: str | None = None,
+        owner_id: str | None = None,
+        status: str = "pending",
+        multitask_strategy: str = "reject",
+        metadata: dict[str, Any] | None = None,
+        kwargs: dict[str, Any] | None = None,
+        error: str | None = None,
+        created_at: str | None = None,
+    ) -> None: ...
+
+    @abc.abstractmethod
+    async def get(self, run_id: str) -> dict[str, Any] | None: ...
+
+    @abc.abstractmethod
+    async def list_by_thread(
+        self,
+        thread_id: str,
+        *,
+        owner_id: str | None = None,
+        limit: int = 100,
+    ) -> list[dict[str, Any]]: ...
+
+    @abc.abstractmethod
+    async def update_status(
+        self,
+        run_id: str,
+        status: str,
+        *,
+        error: str | None = None,
+    ) -> None: ...
+
+    @abc.abstractmethod
+    async def delete(self, run_id: str) -> None: ...
+
+    @abc.abstractmethod
+    async def list_pending(self, *, before: str | None = None) -> list[dict[str, Any]]: ...