feat: emit TicketPurchased event on successful ticket purchase

[CHEF-SAVY]

close #190

purchase_ticket previously returned a ticket ID with no on-chain signal. This adds a TicketPurchased event emitted after the ticket is stored and escrow updated.

Event payload (tktbuy):

• ticket_id — newly assigned ticket ID
• event_id — event the ticket belongs to
• buyer — purchaser address
• amount — total amount paid
• platform_fee — fee deducted by the platform
• organizer_amount — amount added to escrow

Changes:

• events/mod.rs — added TicketPurchased struct following existing event patterns
• lumentix_contract.rs — imported and emitted TicketPurchased at the end of purchase_ticket

All 88 tests pass.

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[martinvibes]

lgtm