feat: add subscription contract migration tooling

[Itodo-S]

Summary

• add a reusable Soroban migration framework under contracts/migrations
• add migration runner, validation, rollback, and history/snapshot artifacts
• document the upgrade and cutover flow for contract deployments

Closes #232

Verification

• bash -n contracts/migrations/lib.sh contracts/migrations/001_blue_green_cutover.sh scripts/run-migration.sh scripts/rollback-migration.sh scripts/validate-migration.sh
• ./scripts/run-migration.sh

Notes

• end-to-end execution requires the Soroban CLI to be installed in the target environment

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