Show Home Assistant sensors on the printer card (#1148, #448)

Binds binary_sensor and reading-carrying sensor entities to a printer and
renders their state on its card, worded by Home Assistant's device_class.
Optional per-sensor alert condition drives a notification on the transition
into the alert state and an opt-in interlock that holds queued prints while
alerting -- a hold with a readable waiting_reason, never a failure, and only
ever on a sensor that was read successfully.

Sensors get their own table rather than a wider entity pattern on SmartPlug:
get_smart_plug_by_printer would otherwise hand the card's power button a door
contact to switch.

The hold is passed to the model matcher directly rather than merged into
busy_printers: _check_auto_drying reads that set as "is currently printing"
and would put an idle-but-held printer down the mid-print drying path.

The notification_providers migration spells its default FALSE, not 0 --
Postgres rejects an integer default for a boolean and _safe_execute swallows
the error.

frontend/src/components/AddNotificationModal.tsx

@@ -45,6 +45,7 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
   const [onStockBreakAlert, setOnStockBreakAlert] = useState(provider?.on_stock_break_alert ?? false);
   const [onPlateClearRequired, setOnPlateClearRequired] = useState(provider?.on_plate_clear_required ?? false);
   const [onBedCooled, setOnBedCooled] = useState(provider?.on_bed_cooled ?? false);
+  const [onHaSensorAlert, setOnHaSensorAlert] = useState(provider?.on_ha_sensor_alert ?? false);
   const [onFirstLayerComplete, setOnFirstLayerComplete] = useState(provider?.on_first_layer_complete ?? false);
 
   // Provider-specific config (scalar fields only — event_priorities is split out
@@ -200,6 +201,7 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
       on_stock_break_alert: onStockBreakAlert,
       on_plate_clear_required: onPlateClearRequired,
       on_bed_cooled: onBedCooled,
+      on_ha_sensor_alert: onHaSensorAlert,
       on_first_layer_complete: onFirstLayerComplete,
     };
 
@@ -635,6 +637,13 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
                   <span className="text-sm text-white">{t('notifications.offline')}</span>
                   <Toggle checked={onPrinterOffline} onChange={setOnPrinterOffline} />
                 </div>
+                <div className="flex items-center justify-between col-span-2">
+                  <div>
+                    <span className="text-sm text-white">{t('notifications.haSensorAlert')}</span>
+                    <span className="text-xs text-bambu-gray ml-1">{t('notifications.haSensorAlertDescription')}</span>
+                  </div>
+                  <Toggle checked={onHaSensorAlert} onChange={setOnHaSensorAlert} />
+                </div>
                 <div className="flex items-center justify-between">
                   <span className="text-sm text-white">{t('notifications.error')}</span>
                   <Toggle checked={onPrinterError} onChange={setOnPrinterError} />
@@ -688,6 +697,7 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
               if (onFirstLayerComplete) enabledEvents.push({ key: 'on_first_layer_complete', label: t('notifications.firstLayerCompleteLabel') });
               if (onPrinterOffline) enabledEvents.push({ key: 'on_printer_offline', label: t('notifications.offline') });
               if (onPrinterError) enabledEvents.push({ key: 'on_printer_error', label: t('notifications.error') });
+              if (onHaSensorAlert) enabledEvents.push({ key: 'on_ha_sensor_alert', label: t('notifications.haSensorAlert') });
               if (onAiFailureDetection) enabledEvents.push({ key: 'on_ai_failure_detection', label: t('notifications.aiFailureDetection') });
               if (onFilamentLow) enabledEvents.push({ key: 'on_filament_low', label: t('notifications.lowFilament') });
               if (onMaintenanceDue) enabledEvents.push({ key: 'on_maintenance_due', label: t('notifications.maintenance') });