mattermost, asterisk: dynamic, collision-safe dedicated coturn ranges

Mattermost's own embedded-coturn fallback hardcoded the same relay range
(49153-49352) for every instance, with no per-instance offset -- running
two Mattermost instances without the shared coturn service (or one
alongside Asterisk's own dedicated coturn, now possible via the prior
commit) would silently reproduce the exact pre-merge collision bug this
repo's coturn history warns about, just among Mattermost instances
instead of Asterisk/Mattermost.

Adds find_free_coturn_range() (lib/common.sh, standalone-mode-stubbed in
both services matching the existing port_in_use/find_free_port
convention): a coturn relay range can't be collision-checked with live
socket scanning the way a single port can -- coturn only opens ports
inside its configured range on demand, so an idle range looks the same
as an unclaimed one to ss/netstat. The only reliable check is reading
what every other coturn-owning service's .env on the box actually
claims (COTURN_MAX_PORT for the shared instance, TURN_MAX_PORT for each
dedicated one) and picking a range starting safely past the highest.

Also adds Mattermost's own opt-out prompt for the shared coturn
preference, matching the one just added to Asterisk (fresh-install-only,
never re-asked on update, same as every other coturn-shape decision in
that file). An update now explicitly preserves its existing dedicated
range from .env rather than silently recomputing a new one.

Verified end-to-end: a shared instance + Asterisk's dedicated coturn +
two independent Mattermost instances, each discovering and avoiding
every range already claimed by the others, land on entirely
non-overlapping port blocks.

lib/common.sh

@@ -805,6 +805,41 @@ find_free_port() {
     eval "$_varname='$_port'"
 }
 
+# find_free_coturn_range MIN_VARNAME MAX_VARNAME RANGE_SIZE [START_PORT]
+# A coturn relay port range can't be collision-checked with port_in_use /
+# find_free_port the way a single fixed port can: coturn only opens ports
+# inside min-port..max-port on demand, per active TURN allocation, so an
+# idle range shows up as nothing listening either way — a live socket scan
+# can't tell two coturn CONFIGS apart. The only reliable check is reading
+# what range every other coturn-owning service on the box actually claims,
+# from its own .env (COTURN_MAX_PORT for the shared instance in
+# ~/docker/coturn/.env, TURN_MAX_PORT for every dedicated per-service coturn
+# — Asterisk's own, each Mattermost instance's, etc., each in that service's
+# own .env). Every service directory keeps its .env at the same top-level
+# path, so one glob covers all of them without needing to know which
+# services exist ahead of time.
+#
+# Writes a RANGE_SIZE-wide block starting safely past the highest claimed
+# max-port back into MIN_VARNAME/MAX_VARNAME. No other coturn on the box at
+# all (fresh install, nothing else uses TURN) leaves it at START_PORT — no
+# collision is possible yet, so there's nothing to shift away from.
+find_free_coturn_range() {
+    local _min_varname="$1" _max_varname="$2" _range_size="${3:-200}" _start="${4:-49152}"
+    local _highest_max=$((_start - 1)) _f _found
+    for _f in "$DOCKER_DIR"/*/.env; do
+        [ -f "$_f" ] || continue
+        _found="$(grep -E '^(COTURN|TURN)_MAX_PORT=' "$_f" 2>/dev/null | tail -1 | cut -d= -f2-)"
+        [[ "$_found" =~ ^[0-9]+$ ]] || continue
+        [ "$_found" -gt "$_highest_max" ] && _highest_max=$_found
+    done
+    local _min=$_start
+    if [ "$_highest_max" -ge "$_start" ]; then
+        _min=$((_highest_max + 50))
+    fi
+    eval "$_min_varname='$_min'"
+    eval "$_max_varname='$((_min + _range_size))'"
+}
+
 # ── Caddy reverse-proxy wiring (shared by every web service) ─────────────────
 # Usage: configure_caddy_for_service "Name" "UPSTREAM" "default-subdomain" ["extra"]
 # UPSTREAM: container:port for caddy_net routing (e.g. "filebrowser:80"),