Bulk-create posts across creators

Create (and optionally schedule) multiple posts across one or more creators in a single request.

Each item reuses the same validation, scheduling (publishAt/expiresAt) and creation path as the single-creator createCreatorPost endpoint, and access is enforced per creatorUserUuid exactly as that endpoint does.

The request returns HTTP 200 whenever the envelope is valid. Each item is reported independently in results (index-aligned with the request posts array) as either a success or an error, so one creator's failure never sinks the whole batch.

<Info>Requires: write:post and write:creator scopes, plus access to each creator (agency assignment/admin, or self).</Info>