feat(map): RSSI coverage circles on dashboard map

[iceice400]

Summary

• Add GET /api/nodes/coverage — JOIN nodes with GPS to packet RSSI aggregates (avg + recent count over configurable window, default 7 days).
• Add Coverage View Leaflet overlay (separate from Topology Links) with RSSI-colored circles sized by packet confidence.
• Show unplotted count in the Node Map header for nodes without coordinates; marker/circle click opens the existing node drawer.

Test plan

• python -m unittest tests.test_node_coverage -v (2 passed)
• Enable Coverage View on dashboard map — circles render with sensible colors/sizes
• Toggle Topology Links independently — both overlays coexist
• Verify unplotted count matches nodes lacking lat/lon
• Click marker/circle opens node drawer
• Spot-check performance with 500+ nodes

AI disclosure

Implemented with Cursor AI assistance; reviewed and tested locally.