test(perf): add k6 load test for verify endpoint (#350)

[daree-dev]

• Rewrite backend/load-test.js to target /v1/verify/public/:wallet (no auth)
• Two scenarios: cached (100 VUs, same wallet) and uncached (100 VUs, unique wallets)
• Thresholds: cached p95<500ms, uncached p95<2000ms, error rate<1%
• Uses k6-summary textSummary; outputs load-test-results.json + load-test-summary.json
• Add verify-perf job to ci.yml: starts backend, installs k6, runs test, uploads artifacts (90d)
• Include verify-perf in all-tests gate

Description

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

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Testing

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• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests passed locally with my changes
• Any dependent changes have been merged and published
• If this PR adds a new directory or key file, I have updated docs/folder-structure.md.

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Additional Notes

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closes #350

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