Add Prometheus metrics endpoint

[Errordog2]

Summary

• Add a Prometheus-compatible GET /metrics endpoint with optional METRICS_SECRET bearer auth.
• Track bounded-label API request counts, accepted credit events, Horizon failures, wallet-signature verification failures, and Prisma query duration observations.
• Add focused unit tests for metrics rendering and request middleware increments, plus Vercel scrape documentation.

Closes #38

Validation

• npm test -- src/services/__tests__/metrics.service.test.ts src/middleware/__tests__/metrics.middleware.test.ts passed: 3 tests.
• npm test passed: 12 files / 47 tests.
• npm run build passed.
• git diff --check passed for changed files.
• Sensitive scan over changed files found no payment/account/private-key markers.

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