feat(nodes): signal health sparklines and RSSI health badge

[iceice400]

Summary

• Extend GET /api/nodes/{id}/metrics_history with optional bucket_minutes → signal_buckets (15-min RSSI/SNR aggregates)
• Optional signal_health config (green_rssi_floor, yellow_rssi_floor, min_packets_per_hour) exposed via GET /api/config
• Node cards: lazy-loaded canvas sparklines (IntersectionObserver) + green/yellow/red health badge from 1-hour rolling RSSI average

Why

Node cards only showed last-heard RSSI. Operators need trend-at-a-glance without opening the drawer. Reuses existing metrics API and SQLite packet history — no schema migration.

Type

• Bug fix
• Feature
• Docs
• Refactor
• UI
• Installer
• Region support
• Hardware change

Testing

• Local unit tests
• Tested on hardware
• Dashboard tested on device

python -m unittest tests.test_signal_buckets tests.test_node_metrics_history -v

Manual checklist (on gateway):

1. Nodes tab → cards with RSSI show sparkline slot + badge after scrolling into view
2. Active node (≥5 pkts/hr) → badge green/yellow/red matches 1h avg RSSI
3. Pass-through node (<5 pkts/hr) → badge hidden (no false red)
4. Sparse beacon node → dashed connector between distant buckets in sparkline
5. Edit signal_health thresholds in local.yaml → badge colour updates after refresh
6. Node drawer still works (raw signal array unchanged)

Hardware: RAK7248 or Pi + HAT · US915

Impact

• Parsing
• Relay
• TX
• Radio driver
• Region logic
• UI + read-only API
• Installer only

Risks: Low — read-only SQL; lazy fetch per visible card limits load. Drawer backward compatible.

AI-assisted?

• Yes