Add staff email client with WebSocket real-time updates

Implements a complete email client for platform staff members:

Backend:
- Add routing_mode field to PlatformEmailAddress (PLATFORM/STAFF)
- Create staff_email app with models for folders, emails, attachments, labels
- IMAP service for fetching emails with folder mapping
- SMTP service for sending emails with attachment support
- Celery tasks for periodic sync and full sync operations
- WebSocket consumer for real-time notifications
- Comprehensive API viewsets with filtering and actions

Frontend:
- Thunderbird-style three-pane email interface
- Multi-account support with drag-and-drop ordering
- Email composer with rich text editor
- Email viewer with thread support
- Real-time WebSocket updates for new emails and sync status
- 94 unit tests covering models, serializers, views, services, and consumers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/platformEmailAddresses.ts

@@ -61,6 +61,7 @@ export interface PlatformEmailAddressListItem {
   email_address: string;
   color: string;
   assigned_user?: AssignedUser | null;
+  routing_mode: 'PLATFORM' | 'STAFF';
   is_active: boolean;
   is_default: boolean;
   mail_server_synced: boolean;
@@ -78,6 +79,7 @@ export interface PlatformEmailAddressCreate {
   domain: string;
   color: string;
   password: string;
+  routing_mode?: 'PLATFORM' | 'STAFF';
   is_active: boolean;
   is_default: boolean;
 }
@@ -88,6 +90,7 @@ export interface PlatformEmailAddressUpdate {
   assigned_user_id?: number | null;
   color?: string;
   password?: string;
+  routing_mode?: 'PLATFORM' | 'STAFF';
   is_active?: boolean;
   is_default?: boolean;
 }