feat: add ntfy notification support

[jhcpeixoto]

Closes #2817

Summary

• add ntfy as a notification channel with server URL and topic configuration
• route ntfy test alerts and monitor notifications through a dedicated provider
• update validation, persistence, OpenAPI schema, and the notification form

Tests

• npm run test:unit:services -- --runTestsByPath test/unit/providers/notifications/ntfyProvider.test.ts test/unit/services/notificationsService.test.ts
• npm run build:openapi
• npm run build (client)
• npm run format-check (server)
• npm run format-check (client)