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/models/__init__.py

@@ -8,12 +8,14 @@ from .model_supply import (  # data-driven model -> toner/drum/waste mapping
     SUPPLY_COLORS,
     CAPACITY_TIERS,
 )
+from .supply_alert import PrinterSupplyAlert  # per-printer toner alert state
 
 __all__ = [
     'Printer',
     'PrinterType',
     'PrinterDriver',
     'ModelSupply',
+    'PrinterSupplyAlert',
     'SUPPLY_TYPES',
     'SUPPLY_COLORS',
     'CAPACITY_TIERS',