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',

plugins/printers/models/supply_alert.py

@@ -0,0 +1,41 @@
+"""Per-printer supply alert state.
+
+The supply poller (flask printers check-toner-alerts) is stateless: it reads
+live levels from Zabbix each run. To alert once per downward crossing (and
+re-arm after a refill) it needs to remember the last tier it alerted for each
+printer + supply. One row per (printerid, supplykey); supplykey is the toner
+color (black/cyan/magenta/yellow) or the raw item name when color is unknown.
+"""
+
+from shopdb.extensions import db
+from shopdb.core.models.base import BaseModel
+
+
+class PrinterSupplyAlert(BaseModel):
+    """Last-alerted tier for one printer supply. lasttier in ok/warning/critical."""
+
+    __tablename__ = 'printersupplyalerts'
+
+    printersupplyalertid = db.Column(db.Integer, primary_key=True)
+    printerid = db.Column(
+        db.Integer,
+        db.ForeignKey('printers.printerid', ondelete='CASCADE'),
+        nullable=False,
+        index=True,
+    )
+    supplykey = db.Column(
+        db.String(64),
+        nullable=False,
+        comment='Toner color, or Zabbix item name when color is unknown',
+    )
+    lasttier = db.Column(
+        db.String(16),
+        nullable=False,
+        default='ok',
+        comment='Last tier alerted: ok, warning, or critical',
+    )
+
+    __table_args__ = (
+        db.UniqueConstraint('printerid', 'supplykey',
+                            name='uq_printersupplyalert_printer_key'),
+    )