feat(webapp): add a local Grafana dashboard for the realtime backend metrics

Provisions a Realtime Native Backend dashboard in the local docker
observability stack: delivery-lag percentiles, live polls by path, the
backstop-delivered alarm, gap-recovery replays and evictions, rows per
emission, held feeds, and resolve health. Works out of the box with
docker:full once INTERNAL_OTEL_METRIC_EXPORTER_ENABLED points the
webapp at the bundled collector.

Also moves the local Grafana default port from 3001 to 4001 so it stops
colliding with common dev-server ports.

Author: ericallam