alerts: per-support-team webhook; printedparts routes low-stock to a chosen team

Support teams gain a webhookurl (migration 7d29 + API + settings-page field), so
a team is a notification target. send_webhook(url=) lets a caller override the
site default with a team's webhook. Printedparts gains a 'alert support team'
setting (printedparts_alert_supportteamid) + selector on its settings page;
low-stock alerts post to that team's webhook, falling back to the site
alert_webhook_url. Email leg unchanged. Same pattern extends to other alerting
plugins (printers low-toner next).

shopdb/core/api/supportteams.py

@@ -64,6 +64,7 @@ def create_support_team():
     team = SupportTeam(
         teamname=data['teamname'],
         teamurl=data.get('teamurl'),
+        webhookurl=data.get('webhookurl'),
         isactive=data.get('isactive', True))
 
     db.session.add(team)
@@ -99,7 +100,7 @@ def update_support_team(team_id: int):
                 f"Support team '{data['teamname']}' already exists",
                 http_code=409)
 
-    for key in ['teamname', 'teamurl', 'isactive']:
+    for key in ['teamname', 'teamurl', 'webhookurl', 'isactive']:
         if key in data:
             setattr(team, key, data[key])