Add load testing suite (k6)

[Fatimasanusi]

Add a load testing suite using k6 to test backend API performance under load. Include tests for: GET /api/rwa with pagination, GET /api/rwa/:contractId, POST /api/rwa, and mixed workloads. Run in CI to prevent performance regressions.
Checklist:

• Install k6 and create test scripts
• Test read endpoints at 100, 500, 1000 RPS
• Test write endpoints at 10, 50, 100 RPS
• Add ramp-up and soak tests
• Add to CI pipeline
• Document performance baseline

[shaarknado]

@shaarknado has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i love to work on this issue

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[drips-wave]

Congratulations, @shaarknado! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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