Move several files into a folder in one request

Move many files into one destination folder. Body: {"sources": ["<rel path>", ...], "destination": "<rel folder>"} (both relative to assets/; an empty destination means the assets/ root).

⚠️ Unlike POST /assets/move, this route never overwrites: the destination is probed with head_object first and a collision is reported, not silently clobbered. copy_object has no if-none-match, so this is a check-then-act — the window is small but real; two tabs moving the same filename concurrently can still lose one. It is strictly safer than the unconditional copy, not a lock.

Returns {"moved": n, "results": [{"source", "status", "erro"?}, ...]} with status in moved | collision | source_missing | same_location | invalid | error.