feat: add daily digest email notification module

- New DailyDigestModule with scheduled summary email for open dashboard items
- Extract StatsService from StatsController for reuse in digest
- Add DailyDigestEnabled, UserEmail, UserPreferredUsername to UserSettings entity
- Sync email/username from OIDC token on each get/update call
- Add dailyDigestEnabled to UserSettingsDto and update API
- Notifications tab in UserSettingsPage with enable toggle and "Jetzt senden" button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-backend/src/app.module.ts

@@ -20,6 +20,7 @@ import { UserSettingsModule } from './user-settings/user-settings.module';
 import { LabelPrintAgentModule } from './label-print-agent/label-print-agent.module';
 import { AgrarmonitorModule } from './agrarmonitor/agrarmonitor.module';
 import { FreigabeModule } from './freigabe/freigabe.module';
+import { DailyDigestModule } from './daily-digest/daily-digest.module';
 import * as path from 'path';
 
 @Module({
@@ -51,6 +52,7 @@ import * as path from 'path';
     LabelPrintAgentModule,
     AgrarmonitorModule,
     FreigabeModule,
+    DailyDigestModule,
   ],
 })
 export class AppModule {}