Revert "Use Cloudflare Turnstile secret env name"

This reverts commit e8eafaf9b9.

.env.example

@@ -22,8 +22,8 @@ JWT_SECRET=super_secret_jwt_key_123
 
 # --- CAPTCHA / Bot Protection (Optional) ---
 # Create keys in Cloudflare Turnstile and set both values in production.
-# When TURNSTILE_SECRET is empty, backend CAPTCHA enforcement is disabled.
-TURNSTILE_SECRET=
+# When TURNSTILE_SECRET_KEY is empty, backend CAPTCHA enforcement is disabled.
+TURNSTILE_SECRET_KEY=
 
 # --- Frontend Configuration (Optional) ---
 # If you need to override the API URL for the frontend