Add k6 load test scripts for Voiceprint verification endpoints (FRE-4930)

- k6 script with P99 latency thresholds (enrollment <500ms, verification <250ms, model retrieval <100ms)
- Configurable 500 req/s sustained throughput for 5 minutes
- Mixed workload scenario + individual endpoint scenarios
- GitHub Actions workflow for automated load testing
- Runner script with environment configuration
- JSON result export for CI artifact collection
- .gitignore entry for load test results

.github/workflows/load-test.yml

@@ -0,0 +1,54 @@
+name: Load Test
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+    inputs:
+      target_rps:
+        description: 'Target requests per second'
+        required: false
+        default: '500'
+      duration:
+        description: 'Test duration'
+        required: false
+        default: '300s'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  load-test-voiceprint:
+    name: Voiceprint Load Test
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    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 Voiceprint load tests
+        run: |
+          cd load-tests/voiceprint
+          ./run.sh mixed
+        env:
+          VOICEPRINT_BASE_URL: ${{ secrets.LOAD_TEST_BASE_URL || 'http://localhost:3000' }}
+          API_TOKEN: ${{ secrets.LOAD_TEST_API_TOKEN || 'test-token' }}
+          TARGET_RPS: ${{ github.event.inputs.target_rps || '500' }}
+          DURATION: ${{ github.event.inputs.duration || '300s' }}
+          ENROLLMENT_P99_MS: '500'
+          VERIFICATION_P99_MS: '250'
+          MODEL_RETRIEVAL_P99_MS: '100'
+
+      - name: Upload results
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: load-test-results-${{ github.sha }}
+          path: load-tests/voiceprint/results/
+          retention-days: 30