feat(backend): 完善 Workflow 与 DeepResearch 的运行链路与持久化能力

功能说明：\n- 重构并增强工作流网关执行链路，支持更完整的 proposal 与 run 调度参数透传\n- 增加 DeepResearch/workflow 运行事件持久化能力并补齐建表 SQL 与查询/保存接口适配\n- 优化 lead agent 与 deep research runner 的中间态处理，增强线程收集与执行链并发控制\n- 调整工作流执行器与任务路由，提升兼容性并完善错误与回放行为\n- 补充回归测试：deep_research 持久化、workflow 并发、定义加载、引擎执行、run 流程等覆盖

offline-backend-20260512/backend/tests/test_deep_research_persistence.py

@@ -454,6 +454,25 @@ async def test_event_payload_round_trip():
         await repos["_engine"].dispose()
 
 
+@pytest.mark.asyncio
+async def test_event_payload_serializes_native_source_datetime():
+    """Source-added events share the source model's native ``published_at``."""
+    repos = await _make_repos()
+    try:
+        repo = repos["events"]
+        published_at = datetime(2026, 9, 2, 10, 9, tzinfo=UTC)
+        await repo.append(
+            session_id="drs_1",
+            job_id="drj_1",
+            event_type="source_added",
+            payload={"source": {"id": "src_1", "published_at": published_at}},
+        )
+        events = await repo.list_after("drj_1", after=0)
+        assert events[0]["payload"]["source"]["published_at"] == published_at.isoformat()
+    finally:
+        await repos["_engine"].dispose()
+
+
 @pytest.mark.asyncio
 async def test_event_seq_independent_per_job():
     repos = await _make_repos()