Dashboard card thumbnail (draft or published PNG)

The 1200x630 card PNG for the library thumbnail, keyed by id and editor-gated exactly like GET /{id}/canvas — so it serves for a draft that was never published (unlike the public /url/ variant, which only serves the published card to link-unfurl crawlers). Prefers the NEWEST draft card (baked best-effort on every editor save / create), else the published card (Dashboard.share['sequence']), else 404. Absence is a normal state — an OG bake never fails or blocks its bake, so a dashboard may legitimately have no card yet; the library falls back to the icon. v3-only: a v2 dashboard 409s on the cross-engine gate.