Add serverless handler for Vercel deployments

[cdlus]

Summary

• add a reusable HTTP handler constructor so the proxy logic can be embedded in different runtimes
• introduce a Vercel serverless entrypoint with rewrites so every request runs through the proxy logic
• document the Vercel deployment flow alongside other hosting options

Testing

• go test ./... -v

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https://chatgpt.com/codex/tasks/task_e_68dee04b54dc8321a1853ca9cf51dcf9

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve original request path in Vercel rewrite

The new serverless deployment rewrites every request to /api/index, but the proxy handler builds the upstream target from r.URL.Path (internal/proxy/passthrough.go around target = p.upstreamOrigin + r.URL.Path). When Vercel applies a rewrite, the function receives the destination path, not the original URL, so every request will be forwarded to https://giscus.app/api/index instead of the requested resource (e.g. /widget). The proxy therefore cannot serve any endpoint when deployed on Vercel. The rewrite needs to pass the original path (for example /api/index$1 or a query parameter) so the handler can reconstruct the upstream URL.

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