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/pyproject.toml

@@ -31,6 +31,14 @@ dependencies = [
     "langchain-google-genai>=4.2.1",
     "langgraph-checkpoint-sqlite>=3.0.3",
     "langgraph-sdk>=0.1.51",
+    "sqlalchemy[asyncio]>=2.0,<3.0",
+    "aiosqlite>=0.19",
+    "alembic>=1.13",
+]
+
+[project.optional-dependencies]
+postgres = [
+    "asyncpg>=0.29",
 ]
 
 [build-system]