Is there a way to customize SQL schema, i.e. add custom constraints or define muti-field primary key?
Not through Strapi itself yet we are looking into this for the v4 (once we have a public RFC ready we will let everyone know to give time to review it).
But you can apply them at the database level manually for most cases and they shouldn’t be broken by Strapi, certain constraints can’t be applied at the database level (specifically unique as an example with the I18N beta) but keys for example shouldn’t cause any issues.
DMehaffy, thanks for the reply, that helped a lot!