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