feat(server): add support for drag-drop image upload (groundwork)

[stanislaw]

No description provided.

[LunaMeerkats]

Most PR #3155 test jobs stop before running their suites. The Linux HTML2PDF and end-to-end logs show the same startup error: manage_assets_command.py imports WildcardEnhancedImage from strictdoc.export.rst, but current main moved it to strictdoc.backend.rst.

Changing that import should let those jobs reach their tests. CodeQL is green, but the functional suites have not run far enough to validate this branch.

[stanislaw]

Hey @LunaMeerkats, thanks for the heads up! The tests are failing expectedly because @mettta and I are integrating @thseiler's work with this separate PR. We have rebased the original work and now need to adapt it to the latest code structure.

Once this PR is integrated, we will do more work on this feature as well as proceed with integrating the other PRs.