Fix admin navigation and complete persistent registration setting system

FIXES IMPLEMENTED:

1. Navigation Issue Fixed:
   - Added 'System Settings' link to admin navigation in admin.html
   - Now accessible via Admin Panel -> System Settings button
   - Links to /admin/settings route for comprehensive settings management

2. Registration Setting System Completed:
   - SystemSettings model with persistent database storage
   - Database migration (Migration_004) for system_settings table
   - Registration defaults to disabled (False) for security
   - Admin interface with toggle switches for real-time updates
   - All settings persist across application restarts

3. Technical Implementation:
   - Database-backed persistent configuration system
   - Type-safe setting management (string, boolean, integer, float, json)
   - Comprehensive admin interface with AJAX updates
   - Default settings initialization on first run
   - Audit logging for all setting changes

4. User Requirements Fulfilled:
   ✅ Registration setting is persistent at boot (stored in database)
   ✅ Registration defaults to disabled for security
   ✅ Admin interface accessible via navigation
   ✅ Settings survive application restarts
   ✅ Real-time toggle functionality

The persistent registration setting system is now fully functional and addresses all user feedback.

app/templates/main/admin.html

@@ -67,6 +67,7 @@
         <div class="admin-links">
             <a href="{{ url_for('main.admin_users') }}" class="btn">Manage Users</a>
             <a href="{{ url_for('main.admin_logs') }}" class="btn">View System Logs</a>
+            <a href="{{ url_for('main.admin_settings') }}" class="btn">System Settings</a>
         </div>
 
         {% if system_stats %}