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/BookingDashboardSummaryDto.cs

@@ -0,0 +1,12 @@
+namespace Fitness_core_BE.DTOs.Booking
+{
+    /// <summary>Backs the Bookings page's Dashboard tab stat-card row.</summary>
+    public class BookingDashboardSummaryDto
+    {
+        public int BookingsToday { get; set; }
+        public decimal RevenueToday { get; set; }
+        public int CheckedInNow { get; set; }
+        public int UpcomingConfirmed { get; set; }
+        public decimal OutstandingUnpaid { get; set; }
+    }
+}