feat: #75 Implement event fund withdrawal for organizers

[jayteemoney]

Summary

• Adds withdraw_funds(event_id) to the event_manager Soroban contract so organizers can collect accumulated ticket-sale revenue after an event concludes
• Introduces an escrow model: funds from purchase_ticket / purchase_tickets are now held by the contract (EventBalance per event) rather than sent directly to the organizer, and released via withdraw_funds
• claim_refund deducts from the escrowed balance so refunds on cancelled events remain functional
• Resolves pre-existing merge-conflict artifacts (duplicate function definitions, conflicting error enum values) that prevented the contract from compiling

Requirements fulfilled

Requirement	Implementation
withdraw_funds(event_id) in event_manager	✅ Added
Only callable by event organizer	✅ event.organizer.require_auth()
Only after event end_date has passed	✅ Error::EventNotEnded guard
Only if event is not cancelled	✅ Error::EventAlreadyCanceled guard
Transfer accumulated funds to organizer wallet	✅ Token transfer from contract escrow
Emit withdrawal event	✅ funds_withdrawn event published
Prevent double withdrawal	✅ DataKey::FundsWithdrawn persistent flag

New storage keys

• DataKey::EventBalance(u32) — escrowed ticket-sale balance per event
• DataKey::FundsWithdrawn(u32) — double-withdrawal guard flag

New error variants

• Error::EventNotEnded = 19
• Error::FundsAlreadyWithdrawn = 20

Test results

running 38 tests
test test::test_withdraw_funds_success ... ok
test test::test_withdraw_funds_before_end_date ... ok
test test::test_withdraw_funds_cancelled_event ... ok
test test::test_withdraw_funds_double_withdrawal ... ok
test test::test_withdraw_funds_nonexistent_event ... ok
test test::test_withdraw_funds_not_organizer ... ok
test test::test_withdraw_funds_zero_balance ... ok
test test::test_withdraw_funds_after_partial_refunds ... ok
... (30 existing tests) ...
test result: ok. 38 passed; 0 failed; 0 ignored

Closes #75

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