Add load testing job to GitHub Actions CI pipeline (FRE-4931)

- Add load-test job to ci.yml that runs after docker-build on push to main
- Create combined load test runner (scripts/load-test/run-all.sh) for all services
- Create k6 load test scripts for api, darkwatch, spamshield, and voiceprint
- Add shared k6 utilities (lib/common.js)
- Update load-test.yml to support all services and report artifacts
- Configure k6 cloud output and P99 threshold validation
- Generate load test report as CI artifact

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -177,3 +177,54 @@ jobs:
         env:
           TF_VAR_hibp_api_key: ${{ secrets.HIBP_API_KEY }}
           TF_VAR_resend_api_key: ${{ secrets.RESEND_API_KEY }}
+
+  load-test:
+    name: Load Test
+    runs-on: ubuntu-latest
+    needs: [lint, typecheck, test, docker-build]
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+    environment: staging
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install k6
+        run: |
+          curl -s https://github.com/grafana/k6/releases/download/v0.50.0/k6-linux-amd64.tar.gz -L | tar xz
+          sudo mv k6 /usr/local/bin/
+          k6 version
+
+      - name: Run combined load tests
+        run: |
+          chmod +x scripts/load-test/run-all.sh
+          ./scripts/load-test/run-all.sh
+        env:
+          LOAD_TEST_BASE_URL: ${{ secrets.LOAD_TEST_BASE_URL || 'http://localhost:3000' }}
+          API_TOKEN: ${{ secrets.LOAD_TEST_API_TOKEN || 'test-token' }}
+          TARGET_RPS: ${{ vars.LOAD_TEST_TARGET_RPS || '500' }}
+          DURATION: ${{ vars.LOAD_TEST_DURATION || '300s' }}
+          K6_CLOUD_TOKEN: ${{ secrets.K6_CLOUD_TOKEN || '' }}
+          K6_CLOUD_PROJECT_ID: ${{ vars.K6_CLOUD_PROJECT_ID || '' }}
+
+      - name: Upload load test report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: load-test-report-${{ github.sha }}
+          path: scripts/load-test/reports/
+          retention-days: 30
+
+      - name: Check P99 thresholds
+        if: always()
+        run: |
+          if [ -f scripts/load-test/reports/threshold-results.json ]; then
+            FAILURES=$(jq -r '.metrics | to_entries[] | select(.value.passes == false) | .key' scripts/load-test/reports/threshold-results.json)
+            if [ -n "$FAILURES" ]; then
+              echo "❌ P99 threshold failures:"
+              echo "$FAILURES"
+              exit 1
+            else
+              echo "✅ All P99 thresholds passed"
+            fi
+          else
+            echo "No threshold results file found"
+          fi