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/Facility/FacilityClassDtos.cs

@@ -0,0 +1,36 @@
+namespace Fitness_core_BE.DTOs.Facility
+{
+    public class FacilityClassDto
+    {
+        public Guid FacilityClassId { get; set; }
+        public Guid FacilityId { get; set; }
+        public string? FacilityName { get; set; }
+        public string Name { get; set; } = string.Empty;
+        public string? Description { get; set; }
+        /// <summary>"Daily" | "Weekly" | "Monthly" | "Custom".</summary>
+        public string RecurrenceType { get; set; } = string.Empty;
+        /// <summary>"HH:mm" local time -- null for Custom.</summary>
+        public string? StartTimeOfDay { get; set; }
+        public int DurationMinutes { get; set; }
+        public int WeekDaysMask { get; set; }
+        public string? MonthDays { get; set; }
+        public DateTime SeriesStartDate { get; set; }
+        public DateTime? SeriesEndDate { get; set; }
+        public int? Capacity { get; set; }
+        public bool IsUnlimitedCapacity { get; set; }
+        public bool IsActive { get; set; }
+        public DateTime CreatedAt { get; set; }
+    }
+
+    /// <summary>A confirmed ad-hoc booking whose window overlaps an active class occurrence on
+    /// the same facility -- surfaced so staff can resolve it via the existing Reschedule flow.</summary>
+    public class SchedulingConflictDto
+    {
+        public Guid BookingId { get; set; }
+        public Guid FacilityId { get; set; }
+        public string? FacilityName { get; set; }
+        public string? GuestName { get; set; }
+        public DateTime StartTime { get; set; }
+        public DateTime EndTime { get; set; }
+    }
+}