Update project slug

Replaces the project’s slug with the supplied value.

The slug must match ^[a-z0-9](?:[a-z0-9-]{0,8}[a-z0-9])?$ — 1–10 characters of lowercase letters, digits, and hyphens, with no leading or trailing hyphen. Invalid formats return 422.

Returns 409 if another active project already owns the slug. Slugs from soft-deleted projects are reusable.

Requires Authorization: Basic base64(projectId:projectSecret).