printers: low-toner alerts with configurable thresholds + support-team routing

Poll Zabbix for toner levels on a schedule and email/webhook on a downward
crossing. Warning fires at or below the warning threshold (default 5%),
critical at the critical threshold (default 0%); both thresholds are settings.
State lives in printersupplyalerts so an alert fires once per crossing and
re-arms after a refill.

Recipients mirror the printedparts pattern: plugin-scoped shopdb users +
roles + free-text emails (falling back to the site alert_recipients), and a
chosen support team's webhook (falling back to the site alert_webhook_url).

- PrinterSupplyAlert model + migration printers0002supplyalerts
- alerttier(remaining, warning, critical) + check_supplies poller
- flask printers check-toner-alerts CLI (run via scheduled task/cron)
- printers alert settings + Low-Toner Alerts settings page
- 7 tests: tier boundaries, once-per-crossing + re-arm, toner-only scope,
  custom thresholds, support-team webhook routing

plugins/printers/services/__init__.py

@@ -6,7 +6,9 @@ from .supply_parts import (
     derivesupplytype,
     derivecolor,
     lookupsupplies,
+    alerttier,
 )
+from .supply_alerts import check_supplies
 from .seed_supplies import seedsupplies
 
 __all__ = [
@@ -15,5 +17,7 @@ __all__ = [
     'derivesupplytype',
     'derivecolor',
     'lookupsupplies',
+    'alerttier',
+    'check_supplies',
     'seedsupplies',
 ]