Public sites perf · Phase 3: optional, vendor-neutral edge cache / CDN support #73

Part of the public-site performance plan (master issue linked above). Depends on Phase 2.

Goal

Let any HTTP cache or CDN sit in front of apps/sites using standard headers only. It stays optional and vendor-neutral, so it works for self-hosters. Correct delivery never depends on it, because Layer 1 (Phase 2) does the real caching.

Modes (EDGE_CACHE_MODE)

Tasks

• Emit headers per mode. Compute tags in apps/sites/proxy.ts from the URL path (t:<team>, t:<team>:docs, t:<team>:page:<path>, ...) and confirm they survive the rewrite. Browser Cache-Control stays max-age=0, must-revalidate.
• Never edge-cache /help/tickets/*, chat, search, form APIs, previews, or the webhook routes.
• Purge adapter interface purge(tags[]), with a generic http adapter: a signed POST of { tags } to EDGE_PURGE_URL with EDGE_PURGE_SECRET.
• Outbox destination for edge purges, with retries and acknowledgments. Order: Layer 1 revalidateTag → edge purge → a second edge purge about 10 seconds later (catches renders that were already in progress).
• Sensitive-data purge events count as complete only after the edge purge is acknowledged.
• RSC / _rsc check: the cache key must include the query string so HTML and RSC payloads are never mixed up. Test client-side navigation behind a CDN that ignores Vary.
• Optional ASSET_PREFIX for serving _next/static from a separate host.
• Optional self-hosted recipe: Caddy (TLS, unchanged) → Varnish with xkey → apps/sites, wired through the http adapter. Documented and opt-in, not part of the default stack.
• Docs: configuration reference for the EDGE_* variables.

Done when

• ttl mode works behind an unmodified CDN or proxy.
• purge mode with the Varnish recipe shows a publish within a few seconds.
• Client-side navigation stays correct.