feat: Implement Load Testing with k6

[jessicanath]

Summary

Sets up k6-based load testing covering all required scenarios and integrates them into CI.

Changes

• load-tests/k6/load-test.js — k6 test with ramp-up, sustained, spike, and endurance scenarios
• load-tests/README.md — instructions for running tests and analysing results
• .github/workflows/load-test.yml — CI workflow running load tests on every push to main

Acceptance Criteria Met

• ✅ Load test scenarios for critical flows (payment processing, history queries)
• ✅ Gradual load increase (ramp-up: 0 → 50 VUs over 2 min)
• ✅ Sustained load testing (100 VUs for 10 min)
• ✅ Spike testing (0 → 500 VUs in 30 s)
• ✅ Endurance testing (50 VUs for 2 h, run separately)
• ✅ Performance bottlenecks identified via custom payment_duration trend metric
• ✅ Capacity planning thresholds defined (p95 < 2 s, p99 < 5 s, error rate < 5%)
• ✅ Automated load testing in CI via .github/workflows/load-test.yml

Closes #351

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