API/Webhook: Missing Product Data for Images

Customer ICT&CO reports that product images are frequently missing or incomplete when retrieved via both Salesbuildr's API and webhooks. Some image URLs (e.g., `inishop.com`) are not accessible externally, resulting in 403 errors due to IP whitelisting, despite rendering correctly within Salesbuildr.

As salesbuildr already self-host these images, add a new field for the self-hosted image URL. This way users can use it as a fallback in case the direct URL is not reachable for any reason.