Delete a folder and all its contents from S3

Delete a folder and everything under it (STAGING). Recursive and irreversible.

The frontend picker has always called this route; it simply did not exist (only POST /assets/folder did), so "Eliminar pasta" was 405-ing.

Plain def, not async def: this walks an unbounded prefix and issues blocking boto3 calls, which on the event loop would starve every other request (docs/claude/10-api-patterns.md). FastAPI runs def in a threadpool.