fix(web): wire DOCS_PROXY_URL into deployed Worker env

[cooper (czxtm)]

Summary

Every /docs/* link on the marketing page (stackpanel.com) currently returns HTTP 503:

$ curl -s https://stackpanel.com/docs/quick-start
{"error":"DOCS_PROXY_URL environment variable is not configured"}

The proxy route at apps/web/src/routes/docs/$.ts + index.ts reads process.env.DOCS_PROXY_URL and short-circuits with that 503 when the binding is missing.

Root cause: The proxy was added on 2026-04-23 (74fca2e "Make environment variables required by default") but apps/web/alchemy.run.ts was never updated to forward the URL into Cloudflare.Vite({ env }). The recent feat(env): build-time secret injection commit established the env-forwarding pattern for BETTER_AUTH_SECRET + Polar values but didn't include this one. So the binding has been absent on every deploy since the route landed — docs.stackpanel.com itself is healthy, only the apex proxy is broken.

Fix: Derive DOCS_PROXY_URL per stage to mirror hostnameFor in apps/docs/alchemy.run.ts, and forward it through the same Cloudflare.Vite({ env }) block the secrets use:

• production → https://docs.stackpanel.com
• staging → https://docs.staging.stackpanel.com
• everything else (PR previews, dev) → https://docs.stackpanel.com (we don't deploy per-PR docs Workers; serving real content beats 503'ing)

The CI memo-bust from #27 (.build-info stamp) will force a fresh upload on push so this actually reaches the deployed Worker.

Test plan

• Wait for the deploy-web workflow to finish on this branch
• curl -sI https://<preview-url>/docs/quick-start returns 200 and the rendered Fumadocs page (not the 503 JSON blob)
• After merge, verify on prod: curl -sI https://stackpanel.com/docs/quick-start returns 200
• Click through the Quick start / Why / Changelog / License footer links from stackpanel.com and confirm each renders
• https://docs.stackpanel.com/... direct hits continue to work unchanged

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Generated by Claude Code

[github-actions]

Preview deployed to pr-30 — https://pr-30.stackpanel.com

[cursor]

PR Summary

Low Risk
Low risk: only adds a new environment binding and simple stage-based URL selection; affects request routing for /docs/* but not auth or data handling.

Overview
Fixes broken /docs/* links on the web Worker by deriving a stage-specific docs origin and exporting it as DOCS_PROXY_URL in the Cloudflare.Vite({ env }) deployment config.

production proxies to https://docs.stackpanel.com, staging proxies to https://docs.staging.stackpanel.com, and all other stages fall back to production docs to avoid 503s when a per-preview docs deploy doesn’t exist.

^{Reviewed by Cursor Bugbot for commit 125f274. Configure here.}