priyansh00soni/Nexus ? reverse-engineered prompt

Build me a notification delivery service that companies can call through an API to send messages to their users by email, in app, or webhook.

I want it to be built for multiple tenants, so each customer gets their own API key and their requests are kept separate. It should handle retries safely, avoid duplicate notifications, and make sure nothing gets dropped if a provider is slow or down. The API should accept a request with an idempotency key, validate it, save the notification, and return fast without waiting for delivery to finish.

Please include background workers for the different channels, a queue for jobs, a database to store notification status, and rate limiting per tenant. I also want basic monitoring with health checks, metrics, and a simple dashboard friendly setup, plus API docs I can use to test the endpoints. If you need to check current docs for any libraries or deployment details, look them up online first.