Add $wgRightsSPDXId with the appropriate spdx code to production settings
Presumably, this'd need to happen in the code in WikimediaMessages that sets the copyright licence in the page footer, but with some magic code so that pages for files, and for extensions which mess around with the content licence (like the Data: namespace on Commons provided by the JsonConfig extension?) sets it back to null?
Thats a good callout I wasn't aware of other places that can re-set the content license, i will be sure to address that if this ticket gets implemented. (Still waiting to see if the license info is necessary in an open patch right now https://gerrit.wikimedia.org/r/c/mediawiki/core/+/555560)