Make Lox open invite endpoint only available to telegram bot

We have deployed Lox's distributor on rdsys-frontend-01.torproject.org and Lox client requests can be made to various Lox server endpoints at rdsys-frontend-01.torproject.org/lox. All except one of these requests requires a user to present a valid Lox credential(s) in order to get the desired response. We would like to limit access to the one endpoint that doesn't require any credentials, rdsys-frontend-01.torproject.org/lox/invite to our telegram bot that is running on polyanthum.torproject.org. In the future, we will likely use a token to limit access instead, but during the testing/alpha phase, limiting access to polyanthum is probably sufficient.