[Backend] -- Add Export Artifact Integrity Checks (Checksum + Expiry)

[Devsol-01]

Description:
Exports (CSV/JSON/PDF) are sensitive and can become stale or tampered with if stored incorrectly. Add integrity controls so exported artifacts are:

• immutable after creation
• verifiable by checksum
• only accessible within their validity window

Implement:

• checksum generation when export is finalized (e.g., SHA-256 stored in metadata)
• store checksum and metadata with export record
• when serving an artifact, verify checksum (or verify at upload/finalization time)
• enforce TTL and automatic cleanup

Optionally:

• sign download URLs
• prevent path traversal by ensuring artifacts are retrieved only by export id

Acceptance Criteria:

• Checksum stored with export metadata.
• TTL enforced for export download.
• Tests verify artifact access after expiry is denied.

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[TheShnider]

@TheShnider has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi I would like to work on this.

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[Ayilojay]

@Ayilojay has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer! I would like to work on this issue

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[MarcusDavidG]

@MarcusDavidG has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can handle this issue and will like to work on it, Please assign thank you.

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[Whiznificent]

@Whiznificent has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

gm maintainer i will like to work on this issue

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[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, can I work on this issue?

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[drips-wave]

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

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