Add versioned export format for encrypted notes

[collinsadi]

Note exports have no format version, so future changes risk silently breaking old backups.

Define a versioned, self-describing export envelope and a migration path for older exports.

Acceptance criteria:

• Exports embed a format version and integrity check.
• Importer accepts all documented versions with migration.
• A corrupted file fails with a clear error, never a partial import.

[Abdulrasaq1515]

@Abdulrasaq1515 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Please assign me, I would do a good job fixing it

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Abdulrasaq1515 to this issue.

[dev-shierly]

@dev-shierly has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi maintainer! I would love to help you with this issue. Given my theoretical knowledge and background, I am quite sure that I am well-equipped to tackle it. Please kindly assign it to me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @dev-shierly to this issue.

[overprodigy]

@overprodigy has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I can take this on. I've gone through the details and will provide a well-tested solution aligned with the project standards.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @overprodigy to this issue.

[drips-wave]

Congratulations, @overprodigy! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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