Stream a published motion-story video (Range-enabled)

Serve the stored MP4 for the token-authenticated PUBLISHED view, keyed by the published sequence. Supports a single HTTP Range (206 partial content) so the <video> preview can seek; a multi-range request falls back to the full body (browser video seeking only issues single ranges). ETag: "{sequence}" + public cache headers; the og:image-style ?v={sequence} cache-buster invalidates on republish. 404 when no clip was exported (a normal state — export is on demand) and for a board with row-level security on, whose whole-dataset clip cannot be scoped to the viewer streaming it. ?aspect=square|portrait streams that clip; omitted, the landscape one (the urls /video-state and the export job hand back already carry it). Auth mirrors the other public /url/ surfaces.