refactor: streamline asset upload pipeline and add variant pagination

[sal94]

• What/Why:
  • Extract upload pipeline into dedicated helpers (quota guard, metadata probe, storage write/rename, DB insert) with stage-aware error mapping to clean 500s.
  • Add base repository helpers and migrate AssetRepository to them; add a paginated variant listing.
  • Expose variant pagination on GET /assets/{asset_id}/variants.

Review focus

• Upload error mapping: stage messages make sense and don’t leak internals; quota rollback is still triggered on failures.
• Variant pagination: limit/offset bounds and ordering (descending by id) fit expected client usage.
• Repo base: no behavioral regressions in asset queries; type safety maintained.