fix: sandbox non-stream response not sent due to @Res() express native mode

- sandbox.controller.ts: non-stream chat path now calls res.json() instead
  of returning the value (which NestJS discards in @Res() library mode)
- ai-gateway.service.ts: 商汤科技 only registers sensenova model, no longer
  overwrites deepseek registration
- .gitignore: add .playwright-mcp/ and gitea-login.png

.gitignore

@@ -13,6 +13,8 @@ out/
 # Environment files
 .env
 .env.local
+.playwright-mcp/
+gitea-login.png
 .env.*.local
 backend/.env
 frontend/.env.local