Summary

• When multiple tickets are purchased in a single order, the refund form now shows checkboxes to select which tickets to refund instead of requiring all tickets to be refunded together
• Adds partial refund amount support to Stripe (via amount parameter) and PayPal (via REFUNDTYPE=Partial and AMT) payment gateways
• Only selected attendees are marked as refunded; remaining tickets stay active
• Single-ticket orders continue to work exactly as before

Fixes #1553

Test plan

• Purchase a single ticket and verify the refund form works as before (no checkboxes, full refund)
• Purchase multiple tickets in one order and verify checkboxes appear on the refund form
• Select a subset of tickets and verify only those are refunded (attendee status changes to refund)
• Verify the remaining tickets in the order stay active (status stays published)
• Select all tickets in a multi-ticket order and verify a full refund is processed
• Test with free tickets (organizer refund without payment transaction)
• Verify refund notification emails are sent correctly for partial refunds

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