Get canvas details

Returns the canvas with its full graph (nodes, edges, inputs, viewport) and media status. For agents: use status.media as the canonical readiness interface. status.media.generation.ready means node images and parent edge videos exist. Inspect status.failed_nodes and status.failed_edges for terminal generation failures and customer-safe error messages before deciding whether to start a new paid attempt. status.nodes.pending remains the backward-compatible count of all incomplete nodes, including failed nodes; status.nodes.failed identifies the terminal subset. status.media.preview.ready means the canvas editor can play every concrete edge using completed base WebM and HEVC derivatives. If status.media.preview.waiting_edges > 0, wait and poll again because the original generation job is still preparing derivatives. If status.media.preview.repairable=true, call POST /v1/collections/{id}/canvases/{canvasId}/repair with dry_run=true before repairing. status.media.variants reports optimized size-variant facts only; base preview repair does not create variants. status.ready, status.generated_ready, and status.preview_ready remain as backward-compatible aliases.