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/base.py

@@ -0,0 +1,40 @@
+"""SQLAlchemy declarative base with automatic to_dict support.
+
+All DeerFlow ORM models inherit from this Base. It provides a generic
+to_dict() method via SQLAlchemy's inspect() so individual models don't
+need to write their own serialization logic.
+
+LangGraph's checkpointer tables are NOT managed by this Base.
+"""
+
+from __future__ import annotations
+
+from sqlalchemy import inspect as sa_inspect
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    """Base class for all DeerFlow ORM models.
+
+    Provides:
+    - Automatic to_dict() via SQLAlchemy column inspection.
+    - Standard __repr__() showing all column values.
+    """
+
+    def to_dict(self, *, exclude: set[str] | None = None) -> dict:
+        """Convert ORM instance to plain dict.
+
+        Uses SQLAlchemy's inspect() to iterate mapped column attributes.
+
+        Args:
+            exclude: Optional set of column keys to omit.
+
+        Returns:
+            Dict of {column_key: value} for all mapped columns.
+        """
+        exclude = exclude or set()
+        return {c.key: getattr(self, c.key) for c in sa_inspect(type(self)).mapper.column_attrs if c.key not in exclude}
+
+    def __repr__(self) -> str:
+        cols = ", ".join(f"{c.key}={getattr(self, c.key)!r}" for c in sa_inspect(type(self)).mapper.column_attrs)
+        return f"{type(self).__name__}({cols})"