test: add k6 load test suite

[MickeeJay]

Closes #391

Summary

No load testing infrastructure existed to validate staging performance under
peak concurrency. This PR introduces a k6 load test suite covering the three
critical endpoints, and a weekly GitHub Actions workflow that enforces P95
latency and error-rate thresholds automatically.

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Root Cause

Without scheduled load tests, regressions in throughput or latency could
reach production undetected. There were no defined P95 or error-rate
thresholds against which staging deployments were validated.

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What Changed

1. k6 load test scripts
Added load test coverage for three endpoints:

Endpoint	Thresholds enforced
deposit	P95 latency · error rate
withdrawal	P95 latency · error rate
metrics	P95 latency · error rate

2. Weekly GitHub Actions workflow
Added a scheduled CI workflow that:

• Runs the k6 suite against staging on a weekly cadence
• Publishes P95 latency and error rate as a job summary on each run

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Testing

Check	Result
Local execution	⚠️ Not run — scheduled workflow, validates on CI push
CI trigger	✅ Weekly schedule via GitHub Actions
Result surface	✅ P95 / error rate published in job summary

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[drips-wave]

@MickeeJay Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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