workers: 固定 placement 到美区，规避部分 colo 出站被拦导致的 403/500

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背景：部署在 Cloudflare Workers 时，Worker 默认会在访问者就近的 colo 执行。
实测在部分 APAC colo（例如 SIN / 部分 TW 线路）下，Worker 出站请求 grok 上游会触发 Cloudflare Challenge（返回“Just a moment...”），导致 upstream 403，最终接口返回 500（upstream_error）。
而当请求落到 LAX 等美区 colo 时，同样请求可以稳定 200。

改动：在 wrangler.toml 增加 placement hint：

[placement]
region = "aws:us-west-2"

让 Worker 执行/出站更倾向美区，从而规避上述 upstream block（已在实际环境验证生效）。

[Copilot]

Pull request overview

This PR addresses production issues where Cloudflare Workers executing from certain APAC colos (e.g., SIN, some Taiwan routes) encounter upstream blocking when requesting the Grok API, resulting in 403/500 errors. The solution pins Worker execution to the US West region where requests succeed reliably.

Changes:

• Added placement configuration to wrangler.toml to pin Worker execution to the aws:us-west-2 region, avoiding upstream blocks from certain APAC colos

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