Review and finalize offline transfer architecture for implementation

Background

Following the discovery work completed in #517490 (closed), this issue aims to finalize the architecture design for offline transfer. This follow-up issue would provide a clean starting point to from the proposed work that has been done.

Acceptance Criteria

• Review the proposed offline transfer architecture with fresh perspective
• Address the three outstanding items identified:
  • Remove export tool from architecture (#536479 (closed))
  • Outline export source architecture (#536631 (closed))
    • Complete discovery work on how to export relations to object storage on the export side
    • See relevant comment from George &8985 (comment 1842142417).
  • Handle placeholder users (#545824 (closed))
    • See thread the problems with user mapping on the architecture doc MR.
• Ensure all work is captured in implementation issues
  • Review existing issues under labels offline transfer:import and offline transfer:export
  • Refine issues as needed
  • Set issue dependencies and blocking relationships
• Update the architecture document details and status