Publish Figure Urls

Record {canonical local figure path → figure blob refs} mappings for a session. omicos-core POSTs this after it background-uploads a figure, so the server can rewrite the local path → a serve url when it serves the trajectory to a cloud client (see get_full_trajectory). The stored trajectory bytes are never touched — only this side table.

Body: {"figures": [{ "canonical_path", "cdn_url"?, # legacy public-figs CDN url "storage_uri"?, # State 2: PRIVATE omicos-drive blob of the COMPRESSED figure "original_uri"?, # State 2: PRIVATE omicos-drive blob of the ORIGINAL (full-res) "compressed"?, # is the inline (storage_uri) copy a compressed one "mime"?, "bytes"?, }, ...]}.

A row is accepted when it has canonical_path AND (cdn_url OR storage_uri). Backward compatible: a legacy payload carrying only cdn_url still works. Only the session owner may publish (same write gate as /append).