cytechmobile/sendium ? reverse-engineered prompt

Build me a self hosted SMS gateway that can sit between my apps and my SMS provider.

I want it to accept messages over a simple HTTP API and over SMPP, then route them out through one or more upstream SMPP connections. It should handle things like sender based and destination based routing, fallback routes, retries when delivery fails, and delivery receipts that get normalized and sent back to my app by webhook. I also want basic controls for throughput limits, queueing, and message priority so it can handle higher volume safely.

Please make it easy to run locally with Docker, and include a clear quick start so I can send a test SMS right away. Add a web API that is easy to inspect, with docs and metrics for monitoring. If you need to check current docs or best practices while building it, look them up online.