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/database/entities/user-settings.entity.ts

@@ -34,4 +34,13 @@ export class UserSettings {
 
   @Column({ type: 'json', nullable: true })
   EmailRecipientHistory!: string[] | null;
+
+  @Column({ type: 'boolean', default: false })
+  DailyDigestEnabled!: boolean;
+
+  @Column({ type: 'varchar', length: 255, nullable: true })
+  UserEmail!: string | null;
+
+  @Column({ type: 'varchar', length: 255, nullable: true })
+  UserPreferredUsername!: string | null;
 }