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/migrations/versions/0002_printersupplyalerts.py

@@ -0,0 +1,47 @@
+"""printers: printersupplyalerts table (toner alert crossing state).
+
+Stores the last tier alerted per (printer, supply) so the poller fires once
+on a downward crossing and re-arms after a refill. Idempotent create.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'printers0002supplyalerts'
+down_revision = 'printers0001anchor'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    if 'printersupplyalerts' in inspector.get_table_names():
+        return
+    op.create_table(
+        'printersupplyalerts',
+        sa.Column('printersupplyalertid', sa.Integer(), primary_key=True),
+        sa.Column('printerid', sa.Integer(), nullable=False),
+        sa.Column('supplykey', sa.String(length=64), nullable=False),
+        sa.Column('lasttier', sa.String(length=16), nullable=False,
+                  server_default='ok'),
+        sa.Column('createddate', sa.DateTime(), nullable=False),
+        sa.Column('modifieddate', sa.DateTime(), nullable=False),
+        sa.Column('isactive', sa.Boolean(), nullable=False,
+                  server_default=sa.true()),
+        sa.ForeignKeyConstraint(['printerid'], ['printers.printerid'],
+                                ondelete='CASCADE'),
+        sa.UniqueConstraint('printerid', 'supplykey',
+                            name='uq_printersupplyalert_printer_key'),
+    )
+    op.create_index('ix_printersupplyalerts_printerid',
+                    'printersupplyalerts', ['printerid'])
+
+
+def downgrade():
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    if 'printersupplyalerts' not in inspector.get_table_names():
+        return
+    op.drop_table('printersupplyalerts')