Update metadata on multiple assets

Updates metadata on multiple assets in one transactional call. Each item carries the target asset id and the per-asset change — different fields can be changed on different assets in the same request. Atomic: any per-item validation failure or unknown / cross-user id rejects the whole batch and writes nothing.

Up to 200 items per request; over-cap requests return 422. For a single-asset edit, prefer update_asset — semantically identical but slightly more concise at the call site.