feat(exports): implement ExportsService and persist job lifecycle

[numdinkushi]

Summary

• Add ExportsService as the owner of export request validation, job enqueueing, and status lookup.
• Persist export records through queued → running → completed | failed with timestamps, and store a delivery reference when complete.
• Wire the export generation handler to update that lifecycle, and document GET /exports/{jobId}.

Fixes #798

Test plan

• POST /exports with a valid payload returns 201, a jobId, and status: queued
• POST /exports with an invalid type, format, or delivery method returns 400
• GET /exports/:jobId returns current status and timestamps; completed responses include deliveryReference
• GET /exports/:jobId for an unknown id returns 404
• Backend unit tests covering enqueue, transitions, and invalid requests pass
• pnpm run lint and pnpm run build succeed in app/backend

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[Cedarich]

@numdinkushi kindly fix CI

[Cedarich]

@numdinkushi