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>

Helpers/TimeZoneHelper.cs

@@ -0,0 +1,34 @@
+namespace Fitness_core_BE.Helpers
+{
+    // Every gym this runs for is Sri Lanka-based -- shared by SubscriptionRenewalJob (daily
+    // trigger wake-up) and the Classes feature (converting a class's local StartTimeOfDay to
+    // UTC per generated occurrence). Lifted out of SubscriptionRenewalJob rather than each
+    // caller re-deriving it.
+    public static class TimeZoneHelper
+    {
+        public static readonly TimeZoneInfo SriLanka = ResolveSriLankaTimeZone();
+
+        private static TimeZoneInfo ResolveSriLankaTimeZone()
+        {
+            try
+            {
+                return TimeZoneInfo.FindSystemTimeZoneById("Sri Lanka Standard Time"); // Windows ID
+            }
+            catch (TimeZoneNotFoundException)
+            {
+                try
+                {
+                    return TimeZoneInfo.FindSystemTimeZoneById("Asia/Colombo"); // IANA ID (Linux/macOS)
+                }
+                catch (TimeZoneNotFoundException)
+                {
+                    // Neither ID resolved (e.g. no tzdata on the host) -- Sri Lanka has kept a
+                    // fixed UTC+5:30 offset with no DST since 2006, so a custom fixed-offset
+                    // zone is a safe fallback rather than failing the whole caller.
+                    return TimeZoneInfo.CreateCustomTimeZone(
+                        "Sri Lanka (fixed +05:30)", TimeSpan.FromMinutes(330), "Sri Lanka Time", "Sri Lanka Time");
+                }
+            }
+        }
+    }
+}