Read canvas generation progress

Generation progress for a canvas. Poll it every 15 to 30 seconds after POST /v1/canvases/{canvasId}/generate-all. Read-only keys can call it, and it never changes the canvas. status.generation.nodes and status.generation.edges count total, completed, pending and failed parts. Pending means neither finished nor failed. Stop polling when status.generation.ready is true: every node image and edge video exists. The canvas plays in the editor once status.preview.ready is also true. If status.preview.waiting_edges is above 0, keep polling because derivatives are still being prepared. If status.preview.repairable is true, call POST /v1/canvases/{canvasId}/repair with dry_run: true first. If any failed count is above 0, read status.failed_nodes and status.failed_edges. Each entry has the part id, its job_id and a customer-safe error. A retry is a new paid attempt, so ask before retrying. Returns 409 when the canvas has no mascot generation context.