fix(site-import): config_json Text 列 JSON 串反序列化（生产走查抓漏——str 传 resolver 炸 .get）

Co-Authored-By: Claude Code <noreply@anthropic.com>

app/platform/routes/site_import_routes.py

@@ -18,6 +18,8 @@
 启动只做校验+建任务——pipeline 由任务模块的后台 aio task 跑，本层不
 感知 crawler/generator（测试也因此能整体打桩 task.start/task.get）。
 """
+import json
+
 from fastapi import APIRouter, Depends, HTTPException, Request
 from pydantic import BaseModel
 from sqlalchemy import select
@@ -45,11 +47,16 @@ async def start_import(slug: str, body: StartRequest, request: Request,
         tenant = await _get_tenant_or_404(session, slug)
         # 租户配置快照（终审 Important #2：AI 三级链的租户槽位——传给
         # task.start → make_provider；自有 ai.* 配置的租户烧自己配额）
-        latest = (await session.execute(
+        # config_json 列为 Text——scalar 返回 JSON 串，须 json.loads 成 dict
+        raw_config = (await session.execute(
             select(TenantConfigVersion.config_json)
             .where(TenantConfigVersion.tenant_id == tenant.id)
             .order_by(TenantConfigVersion.version.desc())
             .limit(1))).scalar_one_or_none()
+        try:
+            latest = json.loads(raw_config) if raw_config else None
+        except (TypeError, json.JSONDecodeError):
+            latest = None
     if tenant.status != "active":
         raise HTTPException(
             status_code=403, detail=f"租户 {slug!r} 已停用，无法发起导入")