Attach an alert channel to a monitor

Adds one alert channel to this monitor's routing set without otherwise touching its configuration, and returns the updated monitor. Deliberately not PUT /monitors/{id}: that body is a full replace, so it re-validates interval and regions against the current plan (a monitor grandfathered below its plan's floor could not be routed at all) and a stale material field would reset the schedule and the per-region streaks. Idempotent — attaching an already-attached channel succeeds and changes nothing. A missing or foreign monitor or channel id returns 404; a channel type outside the plan returns 400.