Add cancellation reason field for student registrations

[Copilot]

This PR implements the ability for parents and administrators to record reasons when students cancel or don't show up for classes.

Changes Made

Database Schema

• Added cancellation_reason TextField to the Order model
• Created migration 0041_order_cancellation_reason.py
• Field is optional (blank=True, null=True) with helpful description

Admin Interface Enhancements

• Added "Has Reason" boolean indicator column in Order list view
• Made cancellation_reason field editable in the admin form
• Enhanced Order admin to show when cancellation reasons are provided

Session Check-in Template Updates

• Added "Reason" column to both "No Shows" and "Cancelled Tickets" tables
• Displays cancellation reason text or "No reason given" if empty
• Added CSS styling for proper column width and readability
• Truncates long reasons to maintain table layout

Benefits

This feature helps administrators and staff:

• Track patterns in student cancellations and no-shows
• Better understand reasons for absences (illness, scheduling conflicts, etc.)
• Improve communication with families about class participation
• Make data-driven decisions about session planning and policies

Backward Compatibility

The implementation is fully backward compatible:

• Existing orders without reasons display "No reason given"
• All new functionality is optional
• No breaking changes to existing workflows

Example Usage

When viewing the session check-in dashboard, staff will now see:

• "Student had flu symptoms" - helps track health-related absences
• "Conflicting soccer practice schedule" - identifies scheduling issues
• "No reason given" - for orders without specified reasons

The feature provides valuable insights while maintaining a clean, intuitive interface.

Fixes #930.

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