feat: email notifications + deadline reminder system
Database:
- notification_preferences table (user_id, tenant_id, reminder days, email/digest toggles)
- notifications table (type, entity link, read/sent tracking, dedup index)
Backend:
- NotificationService with background goroutine checking reminders hourly
- CheckDeadlineReminders: finds deadlines due in N days per user prefs, creates notifications
- Overdue deadline detection and notification
- Daily digest at 8am: compiles pending notifications into one email
- SendEmail via `m mail send` CLI command
- Deduplication: same notification type + entity + day = skip
- API: GET/PATCH notifications, unread count, mark read/all-read
- API: GET/PUT notification-preferences with upsert
Frontend:
- NotificationBell in header with unread count badge (polls every 30s)
- Dropdown panel with notification list, type-colored dots, time-ago, entity links
- Mark individual/all as read
- NotificationSettings in Einstellungen page: reminder day toggles, email toggle, digest toggle
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