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/user-settings/user-settings.controller.ts

@@ -9,12 +9,12 @@ export class UserSettingsController {
 
   @Get()
   async getSettings(@Request() req: any) {
-    return this.userSettingsService.getSettings(req.user.userId);
+    return this.userSettingsService.getSettings(req.user.userId, req.user.email, req.user.preferredUsername);
   }
 
   @Put()
   async updateSettings(@Request() req: any, @Body() body: any) {
-    return this.userSettingsService.updateSettings(req.user.userId, body);
+    return this.userSettingsService.updateSettings(req.user.userId, body, req.user.email, req.user.preferredUsername);
   }
 
   @Get('senders')