feat(stellar): cache transaction fee estimates

[code-0-stella]

feat(stellar): cache transaction fee estimates

Summary

This change reduces the latency of the Stellar payout hot path by caching network fee estimates and reusing them across repeated submissions.

What changed

• Added a short-lived in-memory cache for transaction fee estimates.
• Reused the cached estimate in both the Soroban approval flow and payment submission flow.
• Added regression coverage to verify repeated payments reuse the cached fee.
• Added a lightweight benchmark script and documentation for the change.

Why

Fee estimation was being queried repeatedly during payout-related transaction submission, adding avoidable overhead to the hot path. Caching the latest network fee estimate lowers that cost while still allowing refreshes on a short TTL.

Verification

• Targeted Stellar unit tests passed:
  • npx jest src/modules/stellar/stellar.service.spec.ts --runInBand
• Benchmark output showed:
  • Approximately 12.5% lower elapsed time
  • Approximately 1.24x higher throughput for the cached path

Closes: #1980

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[RUKAYAT-CODER]

kindly resolve conflict