Host the examples on Cloudflare: embed mode, /html, CI route checks

[pietechsolution]

Prepares the example to run as a hosted demo on Cloudflare Workers and to be embedded in documentation pages.

Changes

• Cloudflare Workers with OpenNext. Adds wrangler.jsonc (worker seatlayer-examples, nodejs_compat, no account ids or routes), open-next.config.ts (static assets incremental cache, no bucket needed), public/_headers, and preview, deploy and cf-typegen scripts. npm run build is still a plain next build, so Vercel and npm start are unchanged. Next.js moves from 15.1.6 to 15.5.26 because @opennextjs/cloudflare 1.20 supports the 15.5 line.
• ?embed=1. Set once in app/layout.tsx: a small inline script marks <html> before the first paint, and CSS hides the navigation and the route heading. Works on every route.
• /html. app/html/index.html is the seat picker as a plain HTML page with the CDN script tag and <YOUR_EVENT_KEY> / <YOUR_PUBLIC_KEY> placeholders. app/html/route.ts fills them from NEXT_PUBLIC_SEATLAYER_EVENT_KEY / NEXT_PUBLIC_SEATLAYER_PUBLIC_KEY per request, and shows a setup message when they are missing. onCheckout posts the hold id to /api/hold, like the single event route. Linked from the navigation as "Plain HTML".
• /api/events lists only the configured events. The account-wide listing is gone. Without a secret key the route serves the configured keys with placeholder names; with SEATLAYER_SECRET_KEY it reads each configured event's real name with the server SDK. An empty NEXT_PUBLIC_SEATLAYER_EVENT_KEY_2 / _3 now falls back to the main key instead of producing an empty key. Wording updated in .env.example, the README, the events page and the checkout handoff page.
• CI route checks. CI still builds every push. When the DEMO_EVENT_KEY and DEMO_PUBLIC_KEY repository secrets exist (optional DEMO_SEASON_KEY), it starts the app and runs scripts/check-routes.mjs, which loads every route with and without ?embed=1 in Chromium and fails on an HTTP error, an uncaught exception, a console error, or a visible navigation in embed mode. Forks without the secrets only build.
• README. A new "Hosting on Cloudflare" section at the end. No other README sections changed except one sentence about the events route.

Verification

• npm ci && npm run build from a clean checkout.
• npx opennextjs-cloudflare build succeeds; opennextjs-cloudflare preview (local workerd) returns 200 for every route, /api/events, and /html with the placeholders filled.
• next dev without keys: scripts/check-routes.mjs passes on all 16 route and embed combinations (setup notices, no console errors, no hydration warnings, navigation hidden with ?embed=1).
• With dummy keys against the local worker the chart routes fail the check with a 404 from the API, as expected, which confirms the check catches a broken event.

Nothing was deployed.

Held: merge when the hosted demo is ready.