Ticketing: implement transfer/resale and organizer check-in logic

[josephchimebuka]

Summary

This PR implements two ticket lifecycle capabilities in the ticketing contract:

• Ticket transfer/resale flow via transfer_ticket with ownership updates
• Organizer-driven ticket validation/check-in via check_in with consumed-state enforcement

Issues Closed

Closes #253
Closes #251

What Changed

• Added missing Soroban event macro import so ticketing contract events compile and publish correctly.
• Enforced organizer authorization in check_in:
  • Caller must authenticate
  • Caller must be the organizer of the ticket's event
  • Duplicate check-ins are still rejected
  • Consumed state and check-in record are persisted
• Kept transfer logic aligned with resale/transfer acceptance criteria:
  • Only current holder can transfer
  • Ownership (ticket.holder) is updated on successful transfer
  • Checked-in tickets cannot be transferred
• Updated ticketing tests to reflect organizer-only check-in behavior for the check-in/transfer scenarios.

Acceptance Criteria Mapping

#253 Implement Ticket Transfer and Resale

• transfer_ticket is implemented and enforced
• Ticket ownership mapping (ticket.holder) updates on transfer
• Tickets are transferable before check-in

#251 Implement Ticket Validation and Check-in Logic

• check_in(ticket_id) behavior is implemented
• Ticket consumed state (checked_in, check_in_time) is updated at check-in
• Reuse is prevented: already checked-in tickets fail subsequent check-in attempts

Verification

• Contract crate build/test command run:
  • cargo test -q in contracts/ticketing (build succeeds in current repo wiring)

Notes

• The repository's existing standalone src/test.rs in contracts/ticketing is not currently wired as an active module in this codebase baseline; this PR updates it for behavior parity but does not change baseline test wiring strategy.

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