Backend: Define audit log retention and export

[Cedarich]

Complexity: Medium (150)

Description
apps/backend/src/audit/ and apps/backend/src/admin-audit/ write audit records through interceptors with no retention policy and no export path. The tables grow without bound, and there is no way to hand an auditor a scoped extract.

Acceptance Criteria

• A documented retention window applies per audit record type.
• A scheduled job archives or purges records beyond the window.
• An admin-only export endpoint produces a filtered extract by date range and actor.
• Export runs are themselves audited.
• Purge and archive operations are covered by tests, including the boundary at the retention edge.

_{Wave 8 · source: backend.md}

[oscar24357]

@oscar24357 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello boss, good evening, can you please assign this issue to me.

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[Bug00Joe]

@Bug00Joe has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer, I will like to work on this issue, please assign me.

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