Delete Project Endpoint

Delete a project and all its linked videos asynchronously.

Returns 202 immediately with an empty body; the cascade runs in a background task. Clients poll GET on the project to detect completion: a 404 means the project document is gone (the user-visible signal that "the project is deleted"). Underlying per-video blob/storage cleanup may continue asynchronously for a short while after that point — perform_video_deletion schedules its own sub-tasks for files / folders / frames cleanup, which run after the cascade returns. None of that is user-visible.

Per-video failures abort the cascade and leave the project doc in place so the user's polling eventually times out and they retry.

Note: this is a hard, fully-cascading delete. There is no "preserve videos" mode — for product_testing, a video stripped of its project context (task, analysis) is meaningless to the user. If a future project type ever needs "delete project but keep videos" semantics, add it then with concrete requirements.