feat: #77 Implement discount codes and promotional pricing

[jayteemoney]

Summary

• Adds organizer-controlled discount codes to the event_manager Soroban contract
• Codes reduce ticket prices by a configurable percentage, support optional usage limits and expiration timestamps, and are validated atomically during purchase
• A new purchase_ticket_with_discount entry-point applies the code and emits both ticket_purchased and discount_applied events

Requirements fulfilled

Requirement	Implementation
Discount code storage per event	DataKey::Discount(u32, String) → DiscountCode
create_discount(event_id, code, percentage, max_uses, expiration)	Added; organizer-only
apply_discount validation during purchase	Private apply_discount_code called by purchase_ticket_with_discount
Usage tracking and limits	uses_remaining decremented atomically; rejected when exhausted
Expiration dates for codes	Checked against env.ledger().timestamp()

New public functions

Function	Description
create_discount(event_id, code, percentage, max_uses, expiration)	Create a discount code (organizer only)
get_discount(event_id, code)	Query a code's current state
purchase_ticket_with_discount(buyer, event_id, tier_index, code)	Buy a ticket at a reduced price

Validation

create_discount guards: organizer auth, event not cancelled, percentage 1–100, expiration in the future (if set), no duplicate codes.

purchase_ticket_with_discount guards: all standard purchase checks + code exists, not expired, uses remaining.

New storage / types / errors

DiscountCode struct: code, percentage, max_uses, uses_remaining, expiration

Errors: DiscountNotFound = 24, DiscountExpired = 25, DiscountMaxUsesReached = 26, InvalidDiscountPercentage = 27, DiscountAlreadyExists = 28, InvalidExpiration = 29

Events: discount_created, discount_applied

Test results

running 65 tests
test test::test_create_discount_success ... ok
test test::test_create_discount_invalid_percentage_zero ... ok
test test::test_create_discount_invalid_percentage_over_100 ... ok
test test::test_create_discount_expiration_in_past ... ok
test test::test_create_discount_duplicate_code ... ok
test test::test_create_discount_cancelled_event ... ok
test test::test_get_discount_not_found ... ok
test test::test_purchase_with_discount_price_reduction ... ok
test test::test_purchase_with_discount_100_percent_free ... ok
test test::test_purchase_with_discount_invalid_code ... ok
test test::test_purchase_with_discount_expired_code ... ok
test test::test_purchase_with_discount_max_uses_exhausted ... ok
test test::test_discount_uses_remaining_decrements ... ok
test test::test_discount_unlimited_uses ... ok
test test::test_discount_with_expiration_valid ... ok
... (50 existing tests) ...
test result: ok. 65 passed; 0 failed; 0 ignored

Closes #77

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@jayteemoney Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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