Implement billing functionality and enhance role management

- Introduced billing capabilities, allowing users to request payments for subscription periods (3/6/12 months) with admin approval via Telegram.
- Updated role management to include a `BillingEnabled` property, preventing billing for admin roles.
- Enhanced the `CreateRoleCommand` and `UpdateRoleCommand` to accept billing parameters, ensuring proper handling during role creation and updates.
- Added new endpoints for billing management and integrated billing checks into VPN config creation to enforce payment requirements.
- Updated related services, models, and tests to support the new billing features, ensuring comprehensive coverage and functionality.
- Enhanced documentation to reflect the new billing processes and role management changes.

backend/src/PnvPanel.Infrastructure/Persistence/AppDbContext.cs

@@ -4,6 +4,7 @@ using PnvPanel.Application.Common.Interfaces;
 using PnvPanel.Domain.Activation;
 using PnvPanel.Domain.Apps;
 using PnvPanel.Domain.Audit;
+using PnvPanel.Domain.Billing;
 using PnvPanel.Domain.Configs;
 using PnvPanel.Domain.Inbounds;
 using PnvPanel.Domain.Instructions;
@@ -58,6 +59,10 @@ public class AppDbContext(DbContextOptions<AppDbContext> options)
 
     public DbSet<PricingSettings> PricingSettings => Set<PricingSettings>();
 
+    public DbSet<BillingSettings> BillingSettings => Set<BillingSettings>();
+
+    public DbSet<PaymentRequest> PaymentRequests => Set<PaymentRequest>();
+
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         base.OnModelCreating(modelBuilder);