feat: Add booking control during events

- Add allow_booking_during_event boolean field to events (defaults to false)
- Implement booking_allowed? method to check if tickets can be purchased
- Add event_started? and event_ended? helper methods
- Include new option in event edit form with clear explanation
- Display booking policy status on event show page
- Add visual indicator when booking is disabled during ongoing events
- Update controller to permit new parameter

This allows promoters to control whether attendees can purchase tickets
after an event has started, providing flexibility for different event types.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app/models/event.rb

@@ -70,6 +70,32 @@ class Event < ApplicationRecord
     "Les coordonnées exactes n'ont pas pu être déterminées automatiquement. Une localisation approximative a été utilisée."
   end
 
+  # Check if ticket booking is currently allowed for this event
+  def booking_allowed?
+    return false unless published?
+    return false if sold_out?
+    return false if canceled?
+
+    # Check if event has started and if booking during event is disabled
+    if event_started? && !allow_booking_during_event?
+      return false
+    end
+
+    true
+  end
+
+  # Check if the event has already started
+  def event_started?
+    return false if start_time.blank?
+    Time.current >= start_time
+  end
+
+  # Check if the event has ended
+  def event_ended?
+    return false if end_time.blank?
+    Time.current >= end_time
+  end
+
   private
 
   # Determine if we should perform server-side geocoding