Versioned storage schema and migration path for persistent structs

[IjayAbby]

Description

Fix: SubscriptionData, EscrowAgreement, Card and PaymentChannel are stored directly as contracttype structs in persistent storage. Adding a field to any of them silently breaks deserialization of existing entries after an upgrade, and contract-upgrade has no story for migrating state — only for swapping WASM.

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Related Issue

Closes #1227

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Test Plan

• Tested locally
• Verified expected behavior
• No regressions introduced

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Acceptance Criteria

• Every persisted struct carries a schema version.
• Each contract exposes an admin-gated migrate() that is idempotent and rejects out-of-order versions.
• A test creates v1 state, performs an upgrade, and asserts values survive intact.
• contracts/DEPLOYMENT.md documents the migration procedure and the rollback constraint.

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Checklist

• Code builds successfully
• Tests pass
• Follows project conventions
• No sensitive data exposed

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[IjayAbby]

Hello @Calebux, you closed the issue without merging it, which means I have not received points.