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/persistence/__init__.py

@@ -0,0 +1,13 @@
+"""DeerFlow application persistence layer (SQLAlchemy 2.0 async ORM).
+
+This module manages DeerFlow's own application data -- runs metadata,
+thread ownership, cron jobs, users. It is completely separate from
+LangGraph's checkpointer, which manages graph execution state.
+
+Usage:
+    from deerflow.persistence import init_engine, close_engine, get_session_factory
+"""
+
+from deerflow.persistence.engine import close_engine, get_engine, get_session_factory, init_engine
+
+__all__ = ["close_engine", "get_engine", "get_session_factory", "init_engine"]