feat: handle duplicate payment transaction ids

[heymide]

Pull Request Template

📝 Description

The protocol is now stable with correct backup retention semantics: archived backups are preserved through cleanup cycles and excluded from the active backup list returned by get_backups.

🎯 Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Performance improvement
• Security enhancement
• Other (please describe):

🧪 Testing

• Unit tests pass
• Integration tests pass
• Manual testing completed
• No breaking changes introduced
• Cross-platform compatibility verified
• Edge cases tested

Test Coverage

Test Results:

✅ backup_retention_validation — archived_backups_survive_automatic_cleanup and restore_backup_replaces_* both passing
✅ Full cargo test suite — exit code 0

📋 Contract-Specific Checks

• Soroban contract builds successfully
• WASM compilation works
• Gas usage optimized
• Security considerations reviewed
• Events properly emitted
• Contract functions tested
• Error handling implemented
• Access control verified

🔗 Related Issues

Closes #722
Fixes #
Related to #

📋 Additional Notes

🧪 How to Test

📸 Screenshots (if applicable)

⚠️ Breaking Changes

🔄 Migration Steps (if applicable)

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📋 Reviewer Checklist

Code Review

• Code is readable and well-structured
• Logic is correct and efficient
• Error handling is appropriate
• Security considerations addressed
• Performance impact assessed

Contract Review

• Contract logic is sound
• Gas usage is reasonable
• Events are properly emitted
• Access controls are correct
• Edge cases are handled

Documentation Review

• Code is self-documenting
• Comments explain complex logic
• README updates are clear
• API changes are documented

Testing Review

• Tests cover new functionality
• Tests are meaningful and pass
• Edge cases are tested
• Integration tests work correctly

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[heymide]

Done, Close: #769