Add promo-code-gated offline payments

[halcycon]

Summary

• Add event setting offline_payment_availability (EVERYONE | PROMO_CODE_ONLY) and promo-code flag allows_offline_payment
• Centralise eligibility in OfflinePaymentEligibilityService and enforce it in TransitionOrderToOfflinePaymentHandler
• Expose offline_payment_available on the public order resource and gate checkout UI accordingly; organiser settings/promo forms updated

Test plan

• Enable Offline + Stripe; leave availability as Everyone — offline option visible for all checkouts
• Switch to Promo code only — offline hidden without a promo code; transition API rejects
• Apply ordinary promo code — still no offline option / API rejects
• Apply promo with “Allows offline payment” — offline visible; Stripe still available; transition succeeds
• Discount-none / max-uses-1 access code works as invoice unlock
• Unit: OfflinePaymentEligibilityServiceTest

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