Extend weathermap dashboard with BGP/ports tables, per-site throughput, fix node status query

Fix node.statusQuery colliding legends: multi-neighbor devices previously
sourced from raw per-neighbor BGP session-state series that collide under
one legend, so an arbitrary neighbor won. Replace with a combined
device-up * worst-BGP-session query per device, with an explicit fallback
so zero-BGP devices (access switches) read as neutral/OK.

Add site label via Prometheus relabel (derived from device naming
convention) instead of per-query label_replace(). Add site/device template
variables, BGP sessions table, ports/interfaces table (Grafana merge
transform, not PromQL join), and per-site throughput graphs.

Refs #49

README.md

@@ -367,6 +367,11 @@ only appears as the sample *value* of
 `(device, interface, vlan)`. Per-VNI MAC counts require a PromQL join on `vlan`
 rather than a native `vni` label.
 
+`site` (`dc`/`campus`/`core`) is also not gnmic-exported. It's added as a
+Prometheus `metric_relabel_configs` rule deriving it from `device` via the
+`<area>-<role><n>` naming convention regex (see #49), rather than repeating
+`label_replace()` in every dashboard query.
+
 - Config: `configs/prometheus/prometheus.yml`
 - This is separate from, and does not replace, the existing external Prometheus
   instance — no cutover yet, both run in parallel pending validation