Add Turnstile protection to user actions

app/(frontend)/termine/[slug]/beitreten/page.tsx

@@ -8,6 +8,7 @@ import { combineDateAndTime } from "@/lib/meeting/status";
 import { formatBerlinDate, formatBerlinTime } from "@/lib/calendar/timezone";
 
 export const dynamic = "force-dynamic";
+const turnstileSiteKey = process.env.TURNSTILE_SITE_KEY ?? "";
 
 export const metadata: Metadata = {
   title: "Meeting beitreten",
@@ -42,7 +43,7 @@ export default async function BeitretenPage({ params }: Args) {
         </p>
 
         <div className="mt-10 rounded-[2rem] bg-white/70 p-8 text-left shadow-sm backdrop-blur">
-          <JoinForm slug={slug} />
+          <JoinForm slug={slug} siteKey={turnstileSiteKey} />
         </div>
       </div>
     </section>