perf(onboarding-embed): cache the embed page at the Netlify edge

[hturnbull93]

Why

The join embed on the UK site loaded slowly. /embed/onboarding-form is rendered on demand by a Netlify function and sent no cache headers (cache-status: "Netlify Edge"; fwd=miss), so each uncached hit paid a cold start. Measured: 4.1s on a cold hit, 0.15–0.4s warm. The recent university typeahead (#1159) isn't the cause: it adds a 12 KB JSON file and client-side filtering only.

What

A load in src/routes/embed/onboarding-form/+page.server.ts sets:

• cache-control: public, max-age=0, s-maxage=300, stale-while-revalidate=86400 (Netlify's edge caches for 5 min; browsers still revalidate)
• netlify-vary: query=locale|country|city|source|languages|bg

Netlify keys on the full query string by default, so unknown params (?x=1) would each create a cache entry and re-run the function. Listing the six params the page reads keeps the key to those. They live in CACHE_VARY_PARAMS, with a pointer comment in +page.svelte to keep the two in sync.

The rendered HTML has no per-visitor input: onboarding mode, Turnstile and the referrer-derived source are all resolved client-side, and nothing reads cookies or Accept-Language. The form POST action is unaffected. A deploy clears the cache.

Findings on the deploy preview

• netlify-cdn-cache-control (with or without durable, s-maxage or max-age) gave no hits and never showed a Netlify Durable cache-status line. Cause: the site runs as Netlify Edge Functions (USE_EDGE_FUNCTIONS = true in svelte.config.js), where durable is unsupported. Plain s-maxage in cache-control did give hits (cache-status: hit; ttl=297), so that is what is used.
• The cache is per edge node, so a request can still miss until each node has the page; hit rate climbs with traffic. On a quiet preview it was ~30% cold, rising as nodes warmed.
• Unknown params shared the bare URL's entry (18/25 hits vs 7/25 when cold). Not a rigorous isolation test (one fixed junk value per run), so worth a glance on production.

Test plan

• cache-control and netlify-vary present on the preview response
• Repeat requests return cache-status: "Netlify Edge"; hit; ttl=…
• Production, after deploy: hit ratio on /embed/onboarding-form?country=United%20Kingdom from the UK site
• Form still submits from the embed

prettier and svelte-check pass (0 errors).

🤖 Generated with Claude Code

https://claude.ai/code/session_01Vver2Rh9E4SC5m6ttCpY7V

[netlify]

✅ Deploy Preview for pauseai ready!

Name	Link
🔨 Latest commit	04ebdde
🔍 Latest deploy log	https://app.netlify.com/projects/pauseai/deploys/6ab60c447f18ca0008620d10
😎 Deploy Preview	https://deploy-preview-1162--pauseai.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 69 (no change from production) Accessibility: 91 (no change from production) Best Practices: 100 (no change from production) SEO: 82 (no change from production) PWA: - View the detailed breakdown and full score reports
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.