ci: gate on known advisories in the .NET dependency tree (RB-14)

npm audit --omit=dev gates the shipped frontend bundle; nothing equivalent
existed for the backend, so the entire .NET dependency tree — direct and
transitive — was unscanned (BIO-016 lists it first under "Absent").

The ticket's literal wording would not have worked. `dotnet list package
--vulnerable` is a reporting command: it prints the advisory table and exits
0 regardless. Verified with a throwaway project on System.Net.Http 4.3.0 —
severity High, GHSA-7jgj-8wvc-jh57, exit code 0. A bare `- run: dotnet list
package --vulnerable` would have added a line that reads like coverage in a
compliance review and enforces nothing, which is worse than leaving the gap
visible.

scripts/dotnet-audit.sh runs the scan and matches "has the following
vulnerable packages" — the exact sentence dotnet prints per project on a hit.
One script, two callers (ci.yml and ci-local.sh), so the workflow and the
local gate cannot drift apart.

No severity threshold and no suppression list: picking either before a real
advisory forces the question would be guessing at a policy nobody needs yet.
Secret scanning, BIO-016's other named absence, stays on the checklist.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/ci-local.sh

@@ -29,6 +29,7 @@ step "test (vitest + coverage)";  npm run test:coverage
 step "build --localize (nl+en)";  npx ng build ssp --localize; npx ng build behandelportal --localize
 step "npm audit (shipped deps)";  npm audit --omit=dev
 step "backend format + tests";    ( cd backend && dotnet format BigRegister.slnx --verify-no-changes && dotnet test BigRegister.slnx --filter "Category!=Integration" )
+step "backend dependency audit";  ./scripts/dotnet-audit.sh
 step "showcase snippets drift";   npm run gen:snippets; git diff --exit-code apps/ssp/src/app/showcase/snippets.generated.ts
 step "behaviour spec drift";      npm run gen:behaviour-spec; git diff --exit-code libs/shared/docs/behaviour-spec.mdx
 step "api-client drift";          npm run gen:api; git diff --exit-code libs/shared/src/infrastructure/api-client.ts backend/swagger.json

scripts/dotnet-audit.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# Fail if any NuGet package (direct or transitive) has a known advisory — the .NET half of
+# `npm audit --omit=dev`, which only ever covered the frontend (RB-14/BIO-016).
+#
+# `dotnet list package --vulnerable` is a REPORTING command: it prints the advisory table and
+# still exits 0. Verified against a deliberately vulnerable project — System.Net.Http 4.3.0,
+# GHSA-7jgj-8wvc-jh57, severity High, exit code 0. So `- run: dotnet list package --vulnerable`
+# on its own is a gate that enforces nothing, which is worse than no gate: it reads like
+# coverage in the workflow file. Matching its output is what makes it block.
+#
+# Shared by .github/workflows/ci.yml and scripts/ci-local.sh so the two cannot drift.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+report=$(dotnet list backend/BigRegister.slnx package --vulnerable --include-transitive)
+echo "$report"
+
+# The exact sentence `dotnet list` prints per project when it finds something; the clean case
+# prints "has no vulnerable packages given the current sources" instead.
+if grep -q "has the following vulnerable packages" <<<"$report"; then
+  echo
+  echo "✖ Vulnerable NuGet packages found (table above)." >&2
+  echo "  Transitive hits can be pinned with a direct PackageReference to a patched version." >&2
+  exit 1
+fi