feat: Enhance booking functionality with invoice management and dashboard summary

- Added customerId to guest booking requests for better identification.
- Implemented new endpoints for managing booking invoices and dashboard summaries.
- Created BookingInvoice and BookingDashboardSummary DTOs to support new features.
- Updated Booking and BookingInvoice models to include payment method and timestamps.
- Enhanced BookingService to handle invoice creation upon payment confirmation.
- Modified BookingRepository to support CRUD operations for BookingInvoice.
- Updated TenantDbContext to include BookingInvoice DbSet and relevant configurations.
- Improved error handling and validation for booking completion and payment processes.

DTOs/Booking/BookingDto.cs

@@ -37,6 +37,9 @@ namespace Fitness_core_BE.DTOs.Booking
 
         public string PaymentStatus { get; set; } = string.Empty;
         public decimal AmountCharged { get; set; }
+        /// <summary>"Cash" | "Bank". Null until PaymentStatus becomes "Paid".</summary>
+        public string? PaymentMethod { get; set; }
+        public DateTime? PaidAt { get; set; }
         public string Status { get; set; } = string.Empty;
         public DateTime BookedAt { get; set; }
         public DateTime? CheckedInAt { get; set; }