feat(web): add Cloudflare Turnstile to the public forms

Render the widget on the contact, event registration and both application
forms, and verify the token with Siteverify in each action before anything
is written to the CMS. Verification fails closed. The check is off unless
both TURNSTILE_SITE_KEY and TURNSTILE_SECRET_KEY are set.

Allow challenges.cloudflare.com in script-src and frame-src.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/web/app/entry.server.tsx

@@ -45,8 +45,10 @@ function contentSecurityPolicy(nonce: string): string {
   const mediaOrigin = new URL(STRAPI_URL).origin;
   const directives = [
     "default-src 'self'",
-    `script-src 'self' 'nonce-${nonce}' 'strict-dynamic' https://www.googletagmanager.com`,
+    `script-src 'self' 'nonce-${nonce}' 'strict-dynamic' https://www.googletagmanager.com https://challenges.cloudflare.com`,
     "script-src-attr 'none'",
+    // The Turnstile widget on the public forms renders in Cloudflare's iframe.
+    "frame-src https://challenges.cloudflare.com",
     "style-src 'self' 'unsafe-inline'",
     `img-src 'self' ${mediaOrigin} https://www.google-analytics.com data: blob:`,
     `media-src 'self' ${mediaOrigin} data: blob:`,