Pin the internal auth port to the value nginx bound at startup

/auth grants anonymous admin to any request whose X-Server-Port matches networking.listen.internal, but it read that port off the live config while nginx binds its listeners once at container start and never reloads them, so any path that swaps the running config could move the trusted port without nginx moving with it. Saving networking.listen.internal equal to the external port applied immediately despite the restart-required warning, which handed unauthenticated admin to everything reaching the external port. Snapshot the port at app creation and compare against that instead, and reject a config whose two listeners share a port number, which nginx would refuse to start with anyway.

docs/docs/configuration/advanced/system.md

@@ -293,6 +293,10 @@ networking:
 
 This setting is for advanced users. For the majority of use cases it's recommended to change the `ports` section of your Docker compose file or use the Docker `run` `--publish` option instead, e.g. `-p 443:8971`. Changing Frigate's ports may break some integrations.
 
+The internal and external ports must be different port numbers, and Frigate will refuse to start otherwise. Requests arriving on the internal port are treated as authenticated admins, so pointing both at the same port would remove authentication from the external one.
+
+Nginx binds these ports when it starts, so port changes only take effect after Frigate restarts.
+
 :::
 
 ### Customizing the Nginx configuration