实现 site_interact 加购与购物车校验（Playwright 通道）

探针实测发现 httpx 因 TLS/HTTP2 指纹被 Rakuten 拒认（cookie 有效但
站点不识别 session），site_interact 改为全程 Playwright + storage_state。
add_to_cart 与 verify_cart 已实测可用：从商品页 __INITIAL_STATE__ 抽
purchase 块、调用方补 variant_id 与 choice、POST basketDomain、用 cart
count JSONP API 与 SPA 渲染后的 cart 页校验。enter_checkout 及之后仍
NotImplementedError：Rakuten 对 checkout 要求 session upgrade（重输密码），
是自动 checkout 的硬墙，未实测过墙方案前留接口缝。

SiteInteractor 类持有 Playwright BrowserContext，由 WorkerRunner 实例
持有、container/main 接线、lifespan 管理生命周期。测试侧 23 个用例，
全套件 333 passed，架构边界守住。

附 5 个探针脚本（scripts/probe_*.py），记录实测路径与响应结构。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/trading/main.py

@@ -44,6 +44,7 @@ def build_container() -> TradingContainer:
         from app.trading.worker.evidence import EvidenceStore
         from app.trading.worker.local_db import LocalDB
         from app.trading.worker.runner import WorkerRunner
+        from app.trading.worker.site_interact import SiteInteractor
 
         client = GatewayClient(
             settings.order_gateway_url,
@@ -52,15 +53,18 @@ def build_container() -> TradingContainer:
         )
         local_db = LocalDB(settings.trading_db_path_resolved)
         evidence = EvidenceStore(settings.evidence_path)
+        site = SiteInteractor(auth_session=container.auth_session, settings=settings)
         runner = WorkerRunner(
             settings=settings,
             gateway_client=client,
             local_db=local_db,
             evidence=evidence,
+            site=site,
         )
         container.worker_client = client
         container.worker_local_db = local_db
         container.worker_evidence = evidence
+        container.worker_site = site
         container.worker_runner = runner
 
     return container
@@ -86,10 +90,12 @@ async def lifespan(app: FastAPI):
 
     worker_task: asyncio.Task | None = None
     if container.worker_runner is not None:
-        # 顺序：先开本地 DB（worker 写证据前要先能写库），再起后台任务
+        # 顺序：先开本地 DB 与 SiteInteractor（worker 写证据/发请求前要先就绪），再起后台任务
         assert container.worker_local_db is not None
         assert container.worker_client is not None
+        assert container.worker_site is not None
         await container.worker_local_db.start()
+        await container.worker_site.start()  # type: ignore[union-attr]
         worker_task = asyncio.create_task(
             container.worker_runner.run(), name="trading-worker"
         )
@@ -113,6 +119,8 @@ async def lifespan(app: FastAPI):
                 await worker_task
             except asyncio.CancelledError:
                 pass
+        if container.worker_site is not None:
+            await container.worker_site.close()  # type: ignore[union-attr]
         if container.worker_local_db is not None:
             await container.worker_local_db.close()
         if container.worker_client is not None: