Add SMS test reminder, "which box handled it" note, and a coturn health check

Three follow-ups from live testing on this session's actual VPS:

- tools/pstn-test-check.sh's SMS section printed the Forward-to-URL value
  to configure but never said what to do next — add the "text this DID,
  then watch journalctl -u sms-inbound -f" step right after it.
- docs/pstn-sms-test-checklist.md: the "which box actually handled this"
  question has a simple answer (a DID's inbound routing targets exactly
  one IP:port, so there's no ambiguity to resolve, only a portal setting
  to confirm) — written up so it doesn't need re-deriving. Also fixed the
  --list example to cd into the repo first; ./setup.sh is a relative path
  and silently fails with "command not found" from any other directory,
  confirmed live in this session.
- New tools/coturn-test-check.sh: health-checks the shared coturn instance
  (services/coturn.sh) and every consumer registered against it (Asterisk,
  any number of Mattermost instances) — container/identity, each cached
  consumer credential cross-checked against coturn's own live user
  database (catches the container/volume-recreated-without-db drift case),
  UFW rules for both the TURN port and the relay range, a capacity
  explanation reasoned from the actual port-range math instead of a guess,
  and a real TURN allocation test per consumer via turnutils_uclient —
  the only way to prove credentials + port range + firewall all actually
  work together, not just that each looks right in isolation. Deliberately
  does not attempt a concurrent load test, since that would consume real
  relay ports other services may be actively using.

Verified the turnadmin -l output parsing, UFW rule matching, and the
empty-array-under-set–u loop pattern against mock data before shipping.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H4k6J1qXXyYxhGEgnJaMvn

tools/pstn-test-check.sh

@@ -232,6 +232,12 @@ if [ -f "$SMS_SETTINGS" ]; then
         echo "    ${SMS_FORWARD_URL}"
         echo "    (paste exactly as shown — press SAVE not RETURN on Anveo's SMS tab, then"
         echo "    reopen it to confirm the whole string came back, it's long)"
+        echo ""
+        echo "  Once that's saved: text ${TRUNK_DID:-this DID} from any OTHER phone (not a"
+        echo "  softphone registered to this Asterisk — an outside cell number), then watch:"
+        echo "    journalctl -u sms-inbound -f"
+        echo "  It should land in Sipnetic (or whichever softphone owns that DID/extension)"
+        echo "  within a few seconds. See docs/pstn-sms-test-checklist.md §10 if it doesn't."
     else
         echo "    sms-inbound is installed but no SMS_FORWARD_URL found in $SMS_SETTINGS"
         echo "    — re-run: sudo ./setup.sh sms-inbound"