Add a flag to permanently retire shared coturn without auto-reinstalling it

ensure_coturn_user() auto-installs shared coturn (services/coturn.sh) any
time $DOCKER_DIR/coturn doesn't exist — correct behavior for "first
service that needs TURN", wrong behavior for "an operator deliberately
decided every consumer should run its own dedicated coturn instead and
removed the shared one on purpose". The function had no way to tell those
two states apart, so deleting ~/docker/coturn didn't actually retire it —
the next service to call this function (a Mattermost reinstall, a fresh
Asterisk install) would silently bring it right back.

touch ~/docker/.coturn-retired now short-circuits the function straight to
the existing "no TURN available, caller degrades gracefully" return path,
before it ever looks at install_coturn. Every existing consumer (Asterisk,
Mattermost) already handles that path correctly today — it's exactly what
happens if shared coturn simply fails to install — so this needed no
changes on the consumer side, only closing the gap in the shared function.

Verified with a mock: with the flag present, install_coturn is never
invoked and the function returns empty COTURN_HOST/rc=1 as expected.

lib/common.sh

@@ -1031,6 +1031,21 @@ ensure_coturn_user() {
     local _consumer="$1"
     COTURN_HOST="" COTURN_PORT="" COTURN_USERNAME="" COTURN_PASSWORD=""
 
+    # An operator can permanently retire shared coturn (e.g. deciding every
+    # consumer should run its own dedicated instance instead) by creating
+    # this flag file. Without checking it here first, retiring shared coturn
+    # by just deleting $DOCKER_DIR/coturn wouldn't actually stick — the very
+    # next service that calls this function (a Mattermost reinstall, a fresh
+    # Asterisk install, ...) would silently reinstall it right back, since
+    # the only signal this function otherwise has is "does the directory
+    # exist yet", which looks identical to "never installed" and "removed on
+    # purpose". touch/rm $DOCKER_DIR/.coturn-retired to toggle this.
+    if [ -f "$DOCKER_DIR/.coturn-retired" ]; then
+        log_info "Shared coturn is retired on this box ($DOCKER_DIR/.coturn-retired exists) —"
+        log_info "$_consumer will run its own dedicated coturn instead."
+        return 1
+    fi
+
     if [ ! -d "$DOCKER_DIR/coturn" ]; then
         if declare -F install_coturn >/dev/null 2>&1; then
             log_info "No shared coturn (TURN/STUN) server yet — setting one up for $_consumer..."