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-frontend/src/api/userSettings.ts

@@ -11,6 +11,7 @@ export interface UserSettingsData {
   mailSignatureHtml: string | null;
   defaultLabelTemplateId: number | null;
   emailRecipientHistory: string[] | null;
+  dailyDigestEnabled: boolean;
 }
 
 export interface SenderOption {
@@ -31,4 +32,7 @@ export const userSettingsApi = {
 
   getSenders: () =>
     api.get<SenderOption[]>('/api/user-settings/senders').then((r) => r.data),
+
+  sendDigestNow: () =>
+    api.post<{ ok: boolean; error?: string }>('/api/daily-digest/send-now').then((r) => r.data),
 };