Turn an imported AAF sequence into an editable project, over SSE.

Events, in order:

• progress (stage: one of downloading/parsing/resolving/creating_project)
• done (project_id, missing, missing_count) — the newly created project.

A clip is linked to its source asset when the user validated it (see PUT .../sources), when source_assets overrides it for this call, or when the match is certain on its own (UMID, or a single full-path hit). The rest stay on the timeline without media, so the edit keeps its structure, and are listed in missing.

Not every source has to be linked to convert, so this is deliberately not idempotent: converting a sequence that was already converted — after importing the media that was missing, say — creates another project from the sources resolved now, and leaves the earlier one untouched. The sequence then points at this latest project (that is the one GET .../sources reports as project_ready).