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/api/client.ts

@@ -791,6 +791,8 @@ export interface AppSettings {
   // Prometheus metrics
   prometheus_enabled: boolean;
   prometheus_token: string;
+  // Bed cooled threshold
+  bed_cooled_threshold: number;
 }
 
 export type AppSettingsUpdate = Partial<AppSettings>;
@@ -1402,6 +1404,8 @@ export interface NotificationProvider {
   on_ams_ht_temperature_high: boolean;
   // Build plate detection
   on_plate_not_empty: boolean;
+  // Bed cooled
+  on_bed_cooled: boolean;
   // Print queue events
   on_queue_job_added: boolean;
   on_queue_job_assigned: boolean;
@@ -1452,6 +1456,8 @@ export interface NotificationProviderCreate {
   on_ams_ht_temperature_high?: boolean;
   // Build plate detection
   on_plate_not_empty?: boolean;
+  // Bed cooled
+  on_bed_cooled?: boolean;
   // Print queue events
   on_queue_job_added?: boolean;
   on_queue_job_assigned?: boolean;
@@ -1495,6 +1501,8 @@ export interface NotificationProviderUpdate {
   on_ams_ht_temperature_high?: boolean;
   // Build plate detection
   on_plate_not_empty?: boolean;
+  // Bed cooled
+  on_bed_cooled?: boolean;
   // Print queue events
   on_queue_job_added?: boolean;
   on_queue_job_assigned?: boolean;