Add Collaborator

Add a user as a collaborator. username is a username or a user ID.

Creator-only — listed collaborators cannot promote others. Four rejection paths:

• 404 if the slug or username doesn't resolve.
• 403 FORBIDDEN if the caller isn't the creator.
• 400 if the target is the creator themselves (already implicit).
• 403 KARMA_TOO_LOW if the target has fewer than MIN_KARMA=5 karma — same gate as project creation.
• 409 CONFLICT if they're already a collaborator.

Collaborators get the same file edit + create + delete + project PATCH permissions as the creator (via _can_edit); they do NOT get to delete the project or add/remove other collaborators.

Rate-limited 10/hr per user under project_collab.