diagnostics: System → Diagnostics with a network test probe; RTP_PORT_START/END

The server checks itself (public IP, DNS, local networks, phone ACL, SIP transports, free audio
ports) and says how to fix each problem. A quick or complete network test asks the probe
(apps/probe, own image) to test this server from outside: HTTP/HTTPS, SIP over UDP/TCP/TLS, the
browser phone's /ws, sampled audio ports. The probe only tests the address that asks.
The audio port range comes from RTP_PORT_START/END (default 10000-20000).

docs/admin-guide/README.md

@@ -35,7 +35,7 @@ server is behind one; see [Network](network.md)):
 | 80 | TCP | Only with the bundled Caddy: getting the HTTPS certificate |
 | 5060 | UDP (+TCP) | SIP: voip.ms and desk phones |
 | 5061 | TCP | Only if you turn on SIP over TLS |
-| 10000–20000 | UDP | Call audio (RTP) |
+| 10000–20000 | UDP | Call audio (RTP); another range with `RTP_PORT_START`/`RTP_PORT_END` ([Network](network.md#call-audio-ports)) |
 
 Everything else (the database, the api on 8080, Asterisk's 5038 and 8088) listens on
 127.0.0.1 only. Don't open them.