Embed local MEDIA: image refs as inline base64 at snapshot-build time,
gated by a workspace/attachment-root allow-list, 512KiB cap, image-MIME
allow-list, and magic-byte content validation. Everything else (traversal,
symlink-escape, file://, non-image, SVG, oversize) becomes a static
placeholder. Closes P0 #6174 (public shares no longer emit authenticated
/api/media requests for local paths).
Co-authored-by: webtecnica <webtecnica@users.noreply.github.com>