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/models/supportteam.py

@@ -17,6 +17,9 @@ class SupportTeam(BaseModel):
     supportteamid = db.Column(db.Integer, primary_key=True)
     teamname = db.Column(db.String(100), unique=True, nullable=False)
     teamurl = db.Column(db.Text)  # ServiceNow group deep link, nullable
+    # Teams (or other) webhook this team receives alerts on; alerting plugins
+    # route to it by selecting this team. Nullable = no webhook for this team.
+    webhookurl = db.Column(db.Text)
 
     # Contacts cascade-delete with the team.
     contacts = db.relationship(