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.API/Validators/BillingValidators.cs

@@ -0,0 +1,17 @@
+using FluentValidation;
+using Meezi.API.Models.Billing;
+using Meezi.Core.Constants;
+using Meezi.Core.Enums;
+
+namespace Meezi.API.Validators;
+
+public class SubscribeRequestValidator : AbstractValidator<SubscribeRequest>
+{
+    public SubscribeRequestValidator()
+    {
+        RuleFor(x => x.PlanTier)
+            .Must(PlanPricing.IsBillableOnline)
+            .WithMessage("This plan must be purchased via sales contact.");
+        RuleFor(x => x.Months).InclusiveBetween(1, 12);
+    }
+}