Each scope card now shows a flowchart of its nodes, processes, and
threats, with a distinct shape per type: stadium for entities,
hexagon for boundaries, rectangle for assets, cylinder for data, and
a red hexagon for threats attached via dashed edges to their process
target. The Mermaid source is built on the backend and exposed as a
new `mermaid` field on RiskAssessmentScope; the frontend just renders
it via @probo/ui's MermaidDiagram and shows a copy button + legend.
Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>
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