Implement SEP-24 withdrawal flow in anchor-service

[MissBlue00]

Task

Add SEP-24 interactive withdrawal flow methods to anchor-service.

Details

• createSep24Withdrawal(assetCode, amount, accountId) initiates withdrawal
• Return interactive URL for user to complete on anchor
• Add pollSep24WithdrawalStatus(id) to check completion
• Add Sep24WithdrawalResponse interface

Path

• packages/anchor-service/src/anchor.service.ts
• packages/anchor-service/src/interfaces/

Test

Run pnpm --filter @stellar-pay/anchor-service build to verify.