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>

scripts/load-test/lib/common.js

@@ -0,0 +1,42 @@
+import { Rate, Trend } from 'k6/metrics';
+
+export const errorRate = new Rate('errors');
+
+export function getBaseUrl() {
+  return __ENV.BASE_URL || 'http://localhost:3000';
+}
+
+export function getTargetRps() {
+  return parseInt(__ENV.TARGET_RPS || '500', 10);
+}
+
+export function getDuration() {
+  return __ENV.DURATION || '300s';
+}
+
+export function defaultThresholds(p99ms) {
+  return {
+    http_req_duration: [`p(99)<${p99ms}`],
+    errors: ['rate<0.01'],
+  };
+}
+
+export function checkResponse(res, expectedStatus = 200) {
+  const pass = check(res, {
+    'status is expected': (r) => r.status === expectedStatus,
+    'response time OK': (r) => r.timings.duration < 5000,
+  });
+  if (!pass) {
+    errorRate.add(1);
+  }
+  return pass;
+}
+
+export function randomString(length = 10) {
+  const chars = 'abcdefghijklmnopqrstuvwxyz0123456789';
+  let result = '';
+  for (let i = 0; i < length; i++) {
+    result += chars.charAt(Math.floor(Math.random() * chars.length));
+  }
+  return result;
+}