Publish a batch of releases

Inserts or updates releases for one source, identified by slug or src_… ID on the bare path, or by the org-scoped slug pair. Releases are matched on url: a new URL inserts, a known URL fills empty fields, or, with mode: "upsert-content", overwrites content and media. Re-posting the same batch is safe. Requires a write token, or a publish token bound to this source. A source's first write with its owner's publish token marks it push-fed, so it is no longer polled.