feat: add bed cooled notification after print completes (#378)

Notify users when the print bed cools below a configurable threshold
(default 35°C) after a print finishes, so they know when to remove parts.

- Backend: DB migration, model, schemas, notification template, service
  method, background cooldown monitor (polls every 15s, 30min timeout)
- Frontend: event toggle in provider card/modal, threshold setting in
  Settings > Notifications, i18n keys for all 5 locales
- Tests: 4 backend + 4 frontend tests
- Docs: README, website, wiki updated

frontend/src/components/AddNotificationModal.tsx

@@ -46,6 +46,7 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
   const [onPrinterError, setOnPrinterError] = useState(provider?.on_printer_error ?? false);
   const [onFilamentLow, setOnFilamentLow] = useState(provider?.on_filament_low ?? false);
   const [onMaintenanceDue, setOnMaintenanceDue] = useState(provider?.on_maintenance_due ?? false);
+  const [onBedCooled, setOnBedCooled] = useState(provider?.on_bed_cooled ?? false);
 
   // Provider-specific config
   const [config, setConfig] = useState<Record<string, string>>(
@@ -145,6 +146,7 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
       on_printer_error: onPrinterError,
       on_filament_low: onFilamentLow,
       on_maintenance_due: onMaintenanceDue,
+      on_bed_cooled: onBedCooled,
     };
 
     if (isEditing) {
@@ -486,6 +488,13 @@ export function AddNotificationModal({ provider, onClose }: AddNotificationModal
                   </div>
                   <Toggle checked={onPrintProgress} onChange={setOnPrintProgress} />
                 </div>
+                <div className="flex items-center justify-between col-span-2">
+                  <div>
+                    <span className="text-sm text-white">Bed Cooled</span>
+                    <span className="text-xs text-bambu-gray ml-1">(after print completes)</span>
+                  </div>
+                  <Toggle checked={onBedCooled} onChange={setOnBedCooled} />
+                </div>
               </div>
             </div>