Facility booking-mode redesign, multi-plan subscriptions, and Classes

Implements the facility-classes-booking-dashboard plan (backend):

- Facility.BookingMode (single Scheduled/AdHoc choice) replaced with
  independent SupportsClasses/SupportsBooking flags, so one facility
  can run a class and accept walk-in bookings; a running class now
  blocks walk-in booking for its window on both create and reschedule.
- CustomerPlan.GrantsGymAccess + CustomerPlanClass join table let a
  customer hold one gym-access plan plus any number of class-only
  plans. Replaced the old single-subscription resolution
  (GetActiveSubscriptionByCustomerId/HasAnyActiveSubscriptionAsync)
  with gym-access-aware and class-aware resolvers across booking
  creation/cancellation, plan assignment, and customer self-service.
  Fixed ChangeMembershipPlan/renewal plan-changes, which resolved "the
  subscription to replace" by most-recent-created and would have
  deactivated a member's class subscription instead of their gym one.
- New FacilityClass model: recurring class definitions (Daily/Weekly/
  Monthly/Custom) with lazy occurrence generation (8-week horizon,
  race-safe via a unique index) and proper unlimited-capacity handling.
  "Mark not held" replaces the old bare-cancel: cascades to cancel and
  refund every confirmed booking on the occurrence through the same
  core CancelBooking uses, and frees the slot for walk-in booking.
  Scheduling conflicts (a walk-in booking now overlapping a class) are
  surfaced for staff to resolve via the existing Reschedule flow.
- Renamed the pre-existing "AdHoc" booking-mode terminology to
  "WalkIn" throughout (routes, methods, DTOs, the stored BookingMode
  value) for a clearer, simpler name.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

DTOs/Booking/BookingDto.cs

@@ -4,20 +4,20 @@ namespace Fitness_core_BE.DTOs.Booking
     {
         public Guid BookingId { get; set; }
 
-        /// <summary>Null for an AdHoc booking (no pre-created session).</summary>
+        /// <summary>Null for a WalkIn booking (no pre-created session).</summary>
         public Guid? FacilitySessionId { get; set; }
         public Guid FacilityId { get; set; }
         public string? FacilityName { get; set; }
 
-        /// <summary>"Scheduled" | "AdHoc" -- read off the facility, tells the FE which action set
-        /// applies (e.g. AdHoc bookings can be rescheduled to any free time; Scheduled ones only
-        /// to another session).</summary>
+        /// <summary>"Scheduled" | "WalkIn" -- tells the FE which action set applies (e.g. WalkIn
+        /// bookings can be rescheduled to any free time; Scheduled ones only to another
+        /// session).</summary>
         public string BookingMode { get; set; } = string.Empty;
 
         public DateTime StartTime { get; set; }
         public DateTime EndTime { get; set; }
 
-        /// <summary>Informational headcount, AdHoc only.</summary>
+        /// <summary>Informational headcount, WalkIn only.</summary>
         public int? PartySize { get; set; }
 
         public bool IsGuest { get; set; }