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/supply_parts.py

@@ -10,10 +10,34 @@ and reading the matching part numbers out of the database.
 from typing import Dict, List, Optional
 
 
-# alert thresholds (percent remaining)
+# display thresholds (percent remaining) for the ok/low/critical report badge
 CRITICAL_THRESHOLD = 5
 LOW_THRESHOLD = 10
 
+# toner email-alert tiers (percent remaining). Distinct from the display
+# badge above: a warning email fires at or below TONER_WARNING_THRESHOLD, a
+# critical email at or below TONER_CRITICAL_THRESHOLD (empty).
+TONER_WARNING_THRESHOLD = 5
+TONER_CRITICAL_THRESHOLD = 0
+
+
+def alerttier(remaining: float, warning: float = TONER_WARNING_THRESHOLD,
+              critical: float = TONER_CRITICAL_THRESHOLD) -> str:
+    """Map a toner percent-remaining to an email-alert tier.
+
+    critical at or below the critical threshold (default 0 = empty), warning at
+    or below the warning threshold (default 5), else ok. Both are configurable
+    (printers_alert_critical_threshold / printers_alert_warning_threshold)."""
+    if remaining <= critical:
+        return 'critical'
+    if remaining <= warning:
+        return 'warning'
+    return 'ok'
+
+
+# tier severity rank; an alert fires only when the rank increases (worsens)
+TIER_RANK = {'ok': 0, 'warning': 1, 'critical': 2}
+
 
 def derivesupplytype(name: str) -> str:
     """Map a Zabbix item name to a supply type."""