feat(backend): perform rigorous load testing on Multi-currency Exchange Rate Service

[barry01-hash]

Closes #1119

Summary

Adds infrastructure for rigorous load testing of the Multi-currency Exchange Rate Service (path payment quote endpoint). Includes autocannon-based load tests, configurable scenarios, and a CLI runner.

Changes

New Files

backend/load-tests/config.js — Load test configuration defining:

• Default parameters (duration, connections, pipelining)
• Exchange rate scenarios: quoteSuccess, quoteNotFound, rateLimited
• Throughput/latency thresholds for pass/fail assertions

backend/load-tests/helpers.js — Reusable utilities:

• runAutocannon(url, opts) — programmatic autocannon runner
• formatResults(title, results) — human-readable result formatting
• assertLoadTestThresholds(results, thresholds) — pass/fail assertions on throughput, error rate, and P95 latency

backend/load-tests/exchange-rate-load.test.js — 4 vitest load test scenarios:

1. Endpoint verification — supertest smoke test confirming the route responds
2. Concurrent load (10s, 5 connections) — tests throughput and latency under sustained concurrency
3. Rate limit enforcement (5s, burst of 250 requests) — verifies 429 responses beyond threshold
4. Burst connections (10s, 20 connections) — ensures no crash under high connection count

backend/scripts/load-test-exchange-rate.js — CLI runner for manual/extended load testing:

• Supports targeting any URL (default http://127.0.0.1:3000)
• Configurable duration and connections via DURATION/CONNECTIONS env vars
• Runs all scenarios or a specific named scenario

Modified Files

backend/package.json — Added dev dependency autocannon and npm scripts:

• npm run test:load — runs load test vitest suite
• npm run load:exchange-rate — runs CLI load test runner

Load Test Results

All 4 load tests pass with thresholds:

• ✅ Zero timeouts across all scenarios
• ✅ Zero errors under concurrent/burst load
• ✅ 429 rate limit responses correctly enforced
• ✅ No crashes under 20 simultaneous connections

CLI Usage

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