feat: node-to-server track posting via HTTP

[jonnyspicer]

Summary

• Adds server mode (--server flag) to tracker-host: an aiohttp HTTP server that receives track events from remote nodes via POST requests
• Adds node_agent package (stdlib-only, no extra deps): runs retina-tracker locally on each Raspberry Pi and POSTs track events to the central server
• Adds detection relay for staging/testing without a local blah2 instance
• Wires existing GeolocatorInstance to server-mode NodeRegistry for optional server-side geolocation
• Nodes self-register dynamically by pushing their blah2 config — no server pre-configuration needed

Closes #2

Architecture

Node (Raspberry Pi):
  blah2 → TCP → retina-tracker → stdout JSONL → node_agent → HTTP POST

Server:
  HTTP receive → NodeRegistry → OutputHandler (daily JSONL files)
                              → GeolocatorInstance (optional lat/lon/alt)

API Endpoints

Method	Path	Description
POST	/api/node/{name}/config	Register node, push radar config
POST	/api/node/{name}/tracks	Push track events (JSONL)
GET	/api/nodes	List registered nodes

Test plan

• 44 unit tests passing (config, node_registry, server, node_agent, geolocator integration)
• E2E verified with live data from radar3.retnode.com: 159 track events flowed through full pipeline
• Deploy node_agent to a Pi with local blah2 running (no relay needed)
• Test with geolocator enabled (requires retina-geolocator on server)

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