[NX-202 Issue] Add pip-audit CI enforcement for Python dependency security

This commit integrates pip-audit to enforce vulnerability checks in CI. Dependencies with unresolved HIGH/CRITICAL vulnerabilities will block builds unless explicitly allowlisted. The process is documented, with a strict policy to ensure exceptions are trackable and time-limited.

.github/workflows/docker-release.yml

@@ -27,6 +27,20 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Dependency security gate (pip-audit)
+        run: |
+          python -m pip install --upgrade pip
+          pip install pip-audit
+          pip-audit -r backend/requirements.txt --format json --aliases --output pip-audit-backend.json || true
+          python backend/scripts/pip_audit_gate.py \
+            --report pip-audit-backend.json \
+            --allowlist ops/security/pip-audit-allowlist.json
+
       - name: Resolve version/tag
         id: ver
         shell: bash