Add configurable WebRTC ICE port range

[carlodrift]

NetherNet uses WebRTC, which gathers ICE candidates from the OS ephemeral port range by default. When running behind a firewall or in Docker with host networking, that forces opening a very large UDP range.

This adds an optional session.ice-port-range config with min/max to pin the WebRTC ICE candidate ports:

session:
  ice-port-range:
    min: 0   # 0 = OS ephemeral (default)
    max: 0

[rtm516]

Just to note this will limit the number of users that can join to however many ports you have open

[carlodrift]

Added a note in the config comment on the impact of a small range.

Each in-progress join uses one port from the range (freed once the player is transferred), so the range caps how many players can be joining at the same moment, not the total player count.