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>

Models/CustomerPlan.cs

@@ -63,5 +63,14 @@ namespace Fitness_core_BE.Models
 
         public bool IsActive { get; set; } = true;
         public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        /// <summary>Meaningful only for a plan linked to one or more classes (via
+        /// CustomerPlanClass) -- never consulted for an ordinary plan with no class links,
+        /// which always grants gym access regardless of this flag's stored value. Lets a class
+        /// plan optionally also grant general gym-floor access ("class + gym" as one product)
+        /// instead of class-only. Migration default true (so every pre-existing plan, which has
+        /// no class links, keeps behaving exactly as before); new class plans default this to
+        /// false in the UI.</summary>
+        public bool GrantsGymAccess { get; set; } = true;
     }
 }