Automated bump to version 5.7.0.

modrinth: managed body.md + auto-sync workflow

Adds .modrinth/body.md as the source-of-truth for the Modrinth project body, and a separate workflow that PATCHes the project on every push that touches body.md. Description fixes don't have to wait for a version bump.

Title set once via direct API call; this workflow only manages the body field. The MODRINTH_TOKEN secret used by the release workflow covers this endpoint as well.