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/frontend/routes.js

@@ -20,6 +20,12 @@ export default [
     component: () => import('./views/ZabbixSettings.vue'),
     meta: { requiresAuth: true, requiresAdmin: true, plugin: 'printers' }
   },
+  {
+    path: 'settings/printer-alerts',
+    name: 'printer-alerts',
+    component: () => import('./views/PrinterAlertsSettings.vue'),
+    meta: { requiresAuth: true, requiresAdmin: true, plugin: 'printers' }
+  },
   {
     path: 'printers',
     name: 'printers',