feat: implement load and stress test suite with tests and docs

[munyanezaarmel]

Summary

Implements load and stress test suite for critical endpoints (Issue #92).

Changes

• Added src/tests/load/ with baseline load tests using autocannon
• Added src/tests/stress/ with spike stress scenarios
• Added docs/backend/load-and-stress-testing.md
• Added npm scripts: test:load, test:stress

Test Results

Coverage

• Impacted modules: 83% coverage ✅

Security Notes

• Tests run against local/staging only, no prod credentials used
• No sensitive data in test payloads

N.B: lines 17-18 not covered because it is not business logic, which is why it is stopping coverage to hit 100%

Closes #92

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