feat: add OpenTelemetry metrics with Prometheus-compatible export

[allisson]

Implements observability instrumentation with metrics collection for HTTP requests and business operations. Metrics can be enabled/disabled via METRICS_ENABLED configuration and are exposed at GET /metrics endpoint in Prometheus exposition format for scraping and monitoring.

Major additions:

• internal/metrics package with Provider, HTTP middleware, and business operation decorators
• Prometheus exporter with configurable namespace prefix (METRICS_NAMESPACE)
• Four metric families: http_requests_total, http_request_duration_seconds, operations_total, operation_duration_seconds
• UseCase decorators for auth, secrets, and transit domains tracking operation counts and latencies with domain, operation, and status labels
• HTTP metrics middleware recording request counts and durations by method, path, and status_code
• Runtime disable behavior: when METRICS_ENABLED=false, metrics middleware and /metrics route are not registered

Documentation updates:

• Added docs/operations/monitoring.md with quickstart, metrics contract, Prometheus/Grafana examples, and production hardening guidance
• Added docs/releases/v0.3.0.md release notes
• Updated environment variable defaults and CLI command examples across getting-started, configuration, and operations guides
• Added metrics troubleshooting matrix and feature PR docs consistency checklist to contributing guide
• Updated README with v0.3.0 highlights and monitoring links

Breaking changes: None (additive feature)
Compatibility: API v1 remains stable, metrics endpoint outside versioning