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.

Interfaces/ICustomerRepository.cs

@@ -31,6 +31,9 @@ namespace Fitness_core_BE.Interfaces
         // gym's own schema) -- no clientId parameter needed.
         Task<TenantCustomerLink?> FindActiveLinkByIdentifierAsync(string identifier);
 
+        // Staff picking an existing member to link a walk-in booking to -- identification only.
+        Task<TenantCustomerLink?> GetTenantCustomerLinkById(Guid id);
+
         Task<bool> DeleteCustomer(Guid customerId);
         // Client-initiated delete: deactivates only this customer's presence at the calling
         // client's gym (subscriptions/members/plan assignments scoped to clientId). The shared