PROBLEM ADDRESSED:
User requested: 'the registration setting should be persistent at boot, default disabled'

SOLUTION IMPLEMENTED:

1. SystemSettings Model (app/models.py):
   - Created comprehensive SystemSettings model for persistent configuration storage
   - Added specialized methods: is_registration_enabled(), set_registration_enabled()
   - Implemented type-safe setting management (string, boolean, integer, float, json)
   - Added initialize_default_settings() for automatic setup
   - Comprehensive logging and error handling

2. Database Migration (app/database/migrations.py):
   - Added Migration_004_CreateSystemSettingsTable with MySQL compatibility
   - Automatic creation of system_settings table with proper indexes
   - Default settings insertion (registration_enabled=false, app_name, maintenance_mode)
   - Rollback capability for safe database management

3. Registration Route Integration (app/auth/routes.py):
   - Updated registration route to use SystemSettings.is_registration_enabled()
   - Replaced config-based check with database-backed persistent setting
   - Maintains backward compatibility while adding persistence

4. Admin Interface (app/main/routes.py):
   - Added /admin/settings route for comprehensive system settings management
   - Added /admin/settings/registration for quick registration toggle
   - Added /admin/settings/<setting_key> for individual setting management
   - JSON API endpoints for dynamic frontend updates

5. Admin Template (app/templates/main/admin_settings.html):
   - Modern Bootstrap 5 interface with toggle switches
   - Real-time setting updates via AJAX
   - Quick access cards for registration and maintenance mode
   - Comprehensive settings table with add/edit/delete functionality
   - Modal dialogs for setting management

TECHNICAL BENEFITS:
- ✅ Persistent at boot: Settings stored in database, survive application restarts
- ✅ Default disabled: Registration defaults to False for security
- ✅ Admin control: Full web interface for setting management
- ✅ Type safety: Proper type conversion and validation
- ✅ Audit trail: Comprehensive logging of setting changes
- ✅ Extensible: Easy to add new system settings

SECURITY IMPROVEMENTS:
- Registration disabled by default prevents unauthorized access
- Admin-only setting management with proper authentication
- Type validation prevents injection attacks
- Comprehensive error handling and logging

This fully addresses the user's requirement for persistent, default-disabled registration settings.