Show a persistent admin notification for pending registrations

After further investigation, I found that the backend already supports registration notifications through the registration_alerts_webhook_url field in the InstanceConfigUpdateRequest schema. This field allows the instance to send a webhook notification whenever a new user registers. However, this setting is not exposed in the admin UI, and can only be configured through a direct API call. This creates a gap for self-hosted instance operators who are not developers and cannot easily use the API. Proposed addition to the original request: Expose the registration_alerts_webhook_url setting in the admin UI under the Registration or Notifications section, so it can be configured without API access. Provide a simple UI field where the operator can paste a webhook URL (e.g., a Fluxer webhook, Discord webhook, or any compatible endpoint). Optionally, add a test button next to the field, so the admin can verify the webhook works before saving. This would make the existing backend feature actually usable for non-technical instance operators, and would solve the original problem without requiring any new backend work — only a UI change. Thank you for considering this.