Refactor webhook URL configuration to use environment variable

- Updated questionnaire.js to use a local API endpoint as a fallback for the webhook URL.
- Modified server.js to retrieve the webhook URL from an environment variable.
- Added .env.example file to provide a template for environment configuration, including the optional webhook URL.
- Added an empty responses.json for local storage

.env.example

@@ -0,0 +1,15 @@
+# Environment configuration for the research application
+
+# Port for the web server (optional, defaults to 3000)
+PORT=3000
+
+# External webhook URL for data collection (optional)
+# If not set, data will only be stored locally in data/responses.json
+# Example: WEBHOOK_URL=https://your-domain.com/webhook/endpoint
+WEBHOOK_URL=
+
+# Docker/Traefik configuration (for production deployment)
+YOUR_DOMAIN=your-domain.com
+LETSENCRYPT_EMAIL=your-email@example.com
+TRAEFIK_BASIC_AUTH_USER=admin
+TRAEFIK_BASIC_AUTH_PASSWORD=your-hashed-password