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.controller.ts

@@ -0,0 +1,22 @@
+import { Controller, Post, Request, HttpCode } from '@nestjs/common';
+import { DailyDigestService } from './daily-digest.service';
+
+@Controller('api/daily-digest')
+export class DailyDigestController {
+  constructor(private readonly dailyDigestService: DailyDigestService) {}
+
+  @Post('send-now')
+  @HttpCode(200)
+  async sendNow(@Request() req: any) {
+    const { userId, email, preferredUsername } = req.user;
+    if (!email) {
+      return { ok: false, error: 'Keine E-Mail-Adresse im Benutzerprofil gefunden.' };
+    }
+    try {
+      await this.dailyDigestService.sendDigestForUser(userId, email, preferredUsername);
+      return { ok: true };
+    } catch (err: any) {
+      return { ok: false, error: err.message };
+    }
+  }
+}