One project whiteboard (phoenix_kit_project_whiteboards, chain V5): a
named drawing surface = one core Storage file (the blank-background
bridge — see PhoenixKitProjects.Whiteboards) plus core annotation
rows anchored to that file by PhoenixKitWeb.Components.MediaCanvasViewer.
width/height are duplicated here from the background file so the
list UI can label board sizes without a Storage read. file_uuid is
unique — a board owns its background. created_by_uuid is FK-less
provenance (house convention; activity is the audit trail).