One project whiteboard (phoenix_kit_project_whiteboards, chain V5,
file-less since V16): a named drawing surface of width × height
canvas pixels whose shapes are core annotation rows anchored to
PhoenixKitProjects.Whiteboards.target_type/0 + this row's uuid (core
V183), drawn by PhoenixKitWeb.Components.MediaCanvasViewer in board
mode. file_uuid is nil for such a board; it is set only on boards
from before V16, which keep their background file and its file-anchored
shapes (still unique — a board owned its background). created_by_uuid
is FK-less provenance (house convention; activity is the audit trail).