Backend CI had ruff/mypy static checks and a real pip-audit-equivalent
was missing; frontend already had npm audit, so backend was the only
side of this repo without any dependency-vulnerability gate (partial
coverage). Adds trivy-action fs scoped to backend/, HIGH/CRITICAL only,
consistent with this security programme's gate convention elsewhere.
Verified clean (0 HIGH/CRITICAL) against backend/pyproject.toml before
wiring in, so the gate does not immediately fail CI.
NuklearRabbit
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