Sandboxes reach each other by name on every server

A sandbox can now connect to another sandbox in its organization at <name>.sandbox.internal whichever server runs either one; before, only sandboxes on one server could. Private networks (WireGuard) are reachable from sandboxes on every server too. Connections by name have a daily allowance of 100 GiB for each target sandbox, separate from the private network's 50 GiB. When the private network is briefly unavailable from a server, connections to it return 503 with private-network-unavailable and are never sent anywhere else; retry after a minute.