Set up observability with OpenTelemetry

[Nathydre]

Problem

The backend has OpenTelemetry dependencies (@opentelemetry/sdk-node, prom-client, etc.) but observability setup needs configuration and testing.

Acceptance Criteria

• Configure OpenTelemetry SDK for traces and metrics
• Set up Prometheus metrics endpoint
• Add custom business metrics (payment volume, active users)
• Configure Jaeger exporter for distributed tracing
• Add health check endpoints
• Set up Grafana dashboards
• Document observability setup

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #473)
2. Your PR will be reviewed by the nathydre21 maintainers

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