修复 embed 跨域并持久化圆桌 Step3 结果

- Gateway 默认 CORS 白名单增加 5174，embed 文档补充跨域配置说明
- 圆桌草稿新增 step3 快照字段与迁移，结果绘制 HTML 可随草稿恢复

Co-authored-by: Cursor <cursoragent@cursor.com>

offline-backend-20260512/backend/packages/harness/deerflow/persistence/roundtable_drafts/sql.py

@@ -64,6 +64,7 @@ class RoundtableDraftRepository:
         d = cls._draft_to_meta(row)
         d["step1"] = _loads(row.step1)
         d["step2"] = _loads(row.step2)
+        d["step3"] = _loads(row.step3)
         return d
 
     async def list_drafts(self, user_id: str, *, limit: int = 100) -> list[dict[str, Any]]:
@@ -93,6 +94,7 @@ class RoundtableDraftRepository:
             furthest_step=int(data.get("furthest_step") or 1),
             step1=_dumps(data.get("step1")),
             step2=_dumps(data.get("step2")),
+            step3=_dumps(data.get("step3")),
             created_at=now,
             updated_at=now,
         )
@@ -111,6 +113,7 @@ class RoundtableDraftRepository:
         furthest_step=_UNSET,
         step1=_UNSET,
         step2=_UNSET,
+        step3=_UNSET,
     ) -> dict[str, Any] | None:
         async with self._sf() as session:
             row = await session.get(RoundtableDraftRow, draft_id)
@@ -124,6 +127,8 @@ class RoundtableDraftRepository:
                 row.step1 = _dumps(step1)
             if step2 is not _UNSET:
                 row.step2 = _dumps(step2)
+            if step3 is not _UNSET:
+                row.step3 = _dumps(step3)
             row.updated_at = datetime.now(UTC)
             await session.commit()
             await session.refresh(row)