Add Prometheus /metrics endpoint

[Bortlesboat]

Summary

Add a /metrics endpoint that exposes Prometheus-compatible metrics for monitoring.

Metrics to expose

• satoshi_api_requests_total (counter, by endpoint + status code)
• satoshi_api_request_duration_seconds (histogram)
• satoshi_api_rpc_calls_total (counter, by RPC method)
• satoshi_api_cache_hits_total / satoshi_api_cache_misses_total
• satoshi_api_active_connections (gauge)

Resources

• prometheus_fastapi_instrumentator for easy integration
• Should be opt-in (don't add dependency to base install)

Acceptance criteria

• /metrics endpoint returns Prometheus text format
• Tests verify key metrics appear
• No impact on existing endpoints if prometheus deps not installed