Add a read-only `markers` property on Project that flattens every link's
markers into a single dict keyed by "{link_id}/{name}", each entry
carrying the parent link_id and capture-side node_id. Expose it via
GET /projects/{pid}/markers (Project.Audit) so the frontend can fetch all
markers in one round-trip instead of enumerating links first.
Also surface a marker count in project.stats().
f2360f85fc