Submit Import Job For New Project

Start a background import as a NEW project (Door 1, async).

Returns as soon as the project row and the job exist, so the client can navigate straight to a project that reads as not-yet-ready while the import runs. The county is derived here, from a sample — a sandbox project cannot be created without one, so sampling is what lets the project exist before the resolve does.

Separate from /sandbox/from-features rather than a branch inside it: that route's upload field means "here are the matches I already resolved", and quietly re-pointing it at background execution would strand a browser tab open across the deploy.