The skill-map response gave units -> nodes with id/title/status but no way
to map a node to its lessons, and no other endpoint fills that gap
(/api/progress/nodes doesn't either). Any client rendering the map needs
this to know which lesson to open when a node is tapped.
The ids are already computed in the cached _grade_tree; this just surfaces
them. Additive and backward-compatible; content tests pass.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Zareian
a367685b2e