feat(trading): add read-only Schwab portfolio support

[PatrickXYS]

Adds Schwab portfolio UTA support with token handling and portfolio reads. The broker remains read-only: no order placement or write operations are enabled.\n\nThis branch has been squashed to a single commit as requested.

TESTED locally:

[vercel]

@PatrickXYS is attempting to deploy a commit to the luokerenx4's Team Team on Vercel.

A member of the Team first needs to authorize it.

[PatrickXYS]

@claude review

[luokerenx4]

Thanks for the contribution. When UTA new features are stable, we will review how to support that broker.
Actually in the recent release we have executed the read-only UTA feature. I think it's a good iead. THanks again.

[RainMona]

云端代理审阅（以 upstream dev @ 47e8651b 为准）

结论

延迟 / 需重做：错误 base + 架构已变。

现状

• Base = master（应为 dev）。
• Schwab 仍未进 dev；但 Broker Pack / UTA 模型自 2025-10 以来已大幅变化，直接 rebase 成本极高。
• Checks：Vercel 授权噪声；项目 CI 未见到结果。

建议

1. 若仍要 Schwab：在新分支从当前 dev 按 [docs/broker-packs.md] 重写，不要强推这个 PR。
2. 否则关闭为 stale，等有人按新模型提交。
3. 任何新 PR 必须 base=dev + paper 验收 + review:deep。