Courses: first release checks recorded

Internal: docs/plans/courses-release.md records the read-only SQL the owner ran on the live database on 19 October 2026. No account's public address was taken by the Courses addresses; every new foreign key to users updates in cascade; the six Courses migrations are applied; and the courses and course-media workers are running. ffmpeg and ffprobe are installed, and the signing keys were set in shared/.env. Courses stays behind CoursesPolicy::MEMBERS_ENABLED = false.