feat(backend-sep24): [sep-24] log detailed interaction metrics to prometheus

[GideonBature]

Summary

Adds SEP-24–specific Prometheus metrics for interactive endpoint request duration and volume.

• Registers sep24_interaction_requests_total and sep24_interaction_endpoint_duration_seconds on the shared metrics registry
• Adds sep24MetricsMiddleware to record endpoint, method, status code, and duration on response finish
• Applies middleware to all /sep24 routes (deposit and withdraw interactive endpoints)
• Adds unit tests for metric registration and recording

Closes #618

Test plan

• npm test -- --testPathPattern="sep24-metrics"
• npm test -- --testPathPattern="sep24.route"
• npm run build (fails on pre-existing TS errors in unrelated files: queue-dashboard.route.ts, multisig.service.ts, sep31CallbackNotifier.ts)
• Hit POST /sep24/transactions/deposit/interactive and POST /sep24/transactions/withdraw/interactive, then confirm metrics appear at GET /metrics:
  • sep24_interaction_requests_total{endpoint="...",method="POST",status_code="..."}
  • sep24_interaction_endpoint_duration_seconds{endpoint="...",method="POST",status_code="..."}

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@GideonBature 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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