feat(api): .NET 10 multi-tenant REST API

Full backend implementation:
- Multi-tenant cafe/restaurant management (menus, orders, tables, staff)
- POS order flow with ZarinPal and Snappfood payment integration
- OTP authentication via Kavenegar SMS
- QR digital menu with public discover/finder endpoints
- Customer loyalty, coupons, CRM
- PostgreSQL via EF Core, Redis for caching/sessions
- Background jobs, webhook handlers
- Full migration history

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

src/Meezi.Core/Entities/SystemAdmin.cs

@@ -0,0 +1,8 @@
+namespace Meezi.Core.Entities;
+
+public class SystemAdmin : BaseEntity
+{
+    public string Name { get; set; } = string.Empty;
+    public string Phone { get; set; } = string.Empty;
+    public bool IsActive { get; set; } = true;
+}