Add k6 load testing infrastructure for Darkwatch service

- Create load test directory structure (infra/load-tests/)
- Implement k6 script for Darkwatch endpoints (darkwatch.js)
  - Tests watchlist, scan, exposure, and alert operations
  - Configured for 500 req/s sustained load with P99 < 200ms
  - Includes error rate metrics and threshold validation
- Add documentation and usage guide (README.md)

Related: [FRE-4807](/FRE/issues/FRE-4807)
Co-Authored-By: Paperclip <noreply@paperclip.ing>

infra/load-tests/README.md

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+# ShieldAI Load Tests
+
+k6 load testing suite for ShieldAI services.
+
+## Prerequisites
+
+- k6 v0.45+ installed
+- Target services running on staging environment
+- Authentication tokens for API access
+
+## Running Tests
+
+### Local Execution
+
+```bash
+# Run against local development environment
+k6 run --env BASE_URL=http://localhost:3000 --env AUTH_TOKEN=dev-token src/darkwatch.js
+
+# Run with results output
+k6 run --out json=results.json src/darkwatch.js
+```
+
+### CI/CD Execution
+
+```bash
+# Run on staging environment
+k6 run --env BASE_URL=https://staging-api.freno.me --env AUTH_TOKEN=$STAGING_AUTH_TOKEN src/darkwatch.js
+```
+
+## Test Configuration
+
+Each test script includes:
+
+- **Stages**: Ramp-up, sustained load, ramp-down
+- **Thresholds**: P99 latency and error rate limits
+- **Metrics**: Custom metrics for error tracking
+
+### Current Thresholds
+
+| Service | P99 Latency | Error Rate |
+|---------|-------------|------------|
+| Darkwatch | < 200ms | < 1% |
+
+## Metrics Collection
+
+Run with output options:
+
+```bash
+# JSON output for analysis
+k6 run --out json=darkwatch-results.json src/darkwatch.js
+
+# InfluxDB for visualization
+k6 run --out influxdb=http://influxdb:8086/k6 src/darkwatch.js
+```
+
+## Next Steps
+
+1. Create load test scripts for Spamshield and Voiceprint
+2. Integrate with GitHub Actions CI pipeline
+3. Set up metrics visualization dashboard
+4. Configure alerting on threshold breaches