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/daily-digest/daily-digest.module.ts

@@ -0,0 +1,13 @@
+import { Module } from '@nestjs/common';
+import { DailyDigestService } from './daily-digest.service';
+import { DailyDigestController } from './daily-digest.controller';
+import { StatsModule } from '../stats/stats.module';
+import { UserSettingsModule } from '../user-settings/user-settings.module';
+import { PostprocessingModule } from '../postprocessing/postprocessing.module';
+
+@Module({
+  imports: [StatsModule, UserSettingsModule, PostprocessingModule],
+  providers: [DailyDigestService],
+  controllers: [DailyDigestController],
+})
+export class DailyDigestModule {}