Update a queue entry's subject, body, send/skip status, skip note, or template override

Allows CMs to edit the email subject and body inline before sending, toggle send/skip, record a skip note, or swap the active template. Passing override_template_id triggers a server-side re-render of subject/body using the target template's text plus fresh variable substitution; any rendered_subject/rendered_body passed in the same request are ignored in favor of the re-render. Setting status=skipped records skipped_at and skipped_by_user_id using the authenticated CM. Setting status=pending clears all skip metadata.