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>

docker-compose.yml

@@ -129,6 +129,9 @@ services:
       STRAPI_TOKEN: ${STRAPI_TOKEN:-}
       # Canonical URLs, hreflang alternates and JSON-LD are built from this.
       SITE_URL: ${SITE_URL:-http://localhost:3000}
+      # Cloudflare Turnstile on the public forms - off unless both are set.
+      TURNSTILE_SITE_KEY: ${TURNSTILE_SITE_KEY:-}
+      TURNSTILE_SECRET_KEY: ${TURNSTILE_SECRET_KEY:-}
     volumes:
       # Panton and the Arabic faces are licensed and deliberately excluded from
       # the image (.dockerignore). Mounting the host folder read-only lets a