Closes the v0.1 observability gap. Eleven new metrics in the
dragonfork_webrtc_* namespace (RED-method on the WHEP surface plus
state gauges from the WebRTC subsystem), Prom + Grafana containers
added to deploy/truenas/core/, four pre-loaded alert rules, one
pre-provisioned dashboard.
Hybrid instrumentation: direct client_golang in app/webrtc/ for
hot-path counters and histograms; snapshot collector in
prometheus/webrtc.go for slow-changing gauges. Rationale and
trade-offs against the upstream monitor/metric bus pattern documented
in the Approach section.
Targets v0.2.0-dragonfork.
