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/BookingRequests.cs

@@ -18,6 +18,11 @@ namespace Fitness_core_BE.DTOs.Booking
 
         [Required]
         public string GuestPhone { get; set; } = string.Empty;
+
+        /// <summary>Optional link to an existing registered customer (TenantCustomerLinkId) when
+        /// staff recognizes the walk-in as an already-registered member -- purely for
+        /// identification/auto-fill, doesn't change payment/membership handling at all.</summary>
+        public Guid? CustomerId { get; set; }
     }
 
     public class CreateAdhocBookingRequest
@@ -57,6 +62,10 @@ namespace Fitness_core_BE.DTOs.Booking
         /// the FE pre-fills this from the facility but lets staff adjust it.</summary>
         [Range(typeof(decimal), "0", "79228162514264337593543950335", ErrorMessage = "PricePerHour cannot be negative")]
         public decimal? PricePerHour { get; set; }
+
+        /// <summary>Optional link to an existing registered customer -- see
+        /// CreateGuestBookingRequest.CustomerId.</summary>
+        public Guid? CustomerId { get; set; }
     }
 
     public class MarkGuestBookingPaidRequest
@@ -67,6 +76,20 @@ namespace Fitness_core_BE.DTOs.Booking
         [Required]
         [Range(0.01, double.MaxValue, ErrorMessage = "AmountCharged must be greater than zero")]
         public decimal AmountCharged { get; set; }
+
+        /// <summary>"Cash" | "Bank". Defaults to "Cash" if omitted.</summary>
+        public string? PaymentMethod { get; set; }
+    }
+
+    /// <summary>Body for POST .../complete. Only required when the booking is a guest booking
+    /// still Unpaid -- see BookingService.CompleteBooking. Ignored otherwise (member bookings,
+    /// or a guest booking already Paid).</summary>
+    public class CompleteBookingRequest
+    {
+        public bool? MarkPaid { get; set; }
+        public decimal? AmountCollected { get; set; }
+        /// <summary>"Cash" | "Bank". Defaults to "Cash" if omitted.</summary>
+        public string? PaymentMethod { get; set; }
     }
 
     public class CheckInBookingRequest