feat(backend): refactor legacy codebase in Smart Contract Oracle Integrator

[aetheron06]

Closes #1122
Closes #1123
Closes #1124
Closes #1125

Summary

• Refactor backend/src/lib/transaction-signer.js for clearer validation, replay protection, structured logging, and granular metrics instrumentation.
• Add granular transaction-signer metrics in backend/src/lib/metrics.js:
  • tx_signature_verification_total (outcome)
  • tx_signature_verification_latency_seconds (label)
  • tx_signature_verification_errors_total (error_type)
  • tx_signature_replay_attempts_total
  • tx_signature_validation_failures_total (reason)
  • tx_signature_cache_size
• Add robust merchant-notification caching to backend/src/lib/horizon-poller.js (MerchantConfigCache with LRU eviction and TTL) and use it in batch preload + per-payment fallback.
• Load/robustness coverage is exercised in backend/src/lib/horizon-poller.test.js, including high-volume batch processing and merchant-cache behavior.

Security & performance notes

• Replay cache size is bounded and stale entries are pruned on each verification.
• Merchant config cache reduces DB load and avoids N+1 lookups during poll cycles.
• Metrics are recorded on validation failures, replay attempts, verifier exceptions, and successful/failed outcomes for observability.

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