fix: add missing source parameter

Without source="media" a POST request to the "episodes" endpoint fails when mediaId is set.

This didn't come up so far, because Dashboard only uses updates and Django does some magic when setting model attributes on existing instances.

added bug label

assigned to @eigenwijsje

requested review from @eigenwijsje

approved this merge request

Looks good to me, thanks!

merged

mentioned in commit a2e7a51b