[CON-01] Complete `payment_escrow` Soroban smart contract

[yusuftomilola]

Overview

The payment_escrow contract at contracts/payment_escrow/ is a skeleton. It is responsible for holding funds for monthly/quarterly/yearly bookings and releasing or refunding them — the core escrow lifecycle that the NestJS backend depends on.

Context

• Language: Rust 2021 edition, Soroban SDK (no_std)
• Shared types in contracts/common_types/src/types.rs
• The access_control contract governs who can trigger releases and refunds
• Backend calls this contract via @stellar/stellar-sdk in backend/src/payments/providers/soroban-escrow.provider.ts

Tasks

• Implement create_escrow(booking_id, amount, member_address, hub_address, unlock_date) — creates escrow, stores state in contract storage
• Implement release(escrow_id, caller) — callable only by hub address or access_control admin; transfers funds to hub address
• Implement refund(escrow_id, caller) — callable only before unlock_date or when a cancellation flag is set; returns funds to member
• Implement get_escrow(escrow_id) — read-only view of escrow state (amount, status, parties)
• Add contract tests in payment_escrow/src/test.rs covering: create, release, refund, unauthorised access rejection

Files to Modify / Create

• contracts/payment_escrow/src/ (all source files)

[zakkiyyat]

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[grantfox-oss]

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Next steps:

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@zakkiyyat's PR #1240 was approved and merged by @yusuftomilola.

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