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>

bun.lock

@@ -11,6 +11,7 @@
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+        "slug": "^11.0.0",
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@@ -545,6 +546,8 @@
 
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+
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