feat: real-time updates over SSE across the app + live map (brightness, animated cluster ratio, hover card)

[SimmerV]

Real-time SSE platform + live map

Replaces poll-driven refreshes with a push model: one server-sent-events stream
feeds live node, chat, packet, and telemetry updates to every page, and the map
now reacts to that stream instead of being a static load-time snapshot.

Backend — SSE fan-out

• broadcaster.py (new): in-process pub/sub hub. One bounded asyncio.Queue
  per subscriber; publish() is synchronous and never awaits, so the MQTT ingest
  path can emit without ever blocking on a slow client. Backpressure drops the
  oldest item per slow subscriber.
• GET /v1/events (api/api.py): text/event-stream route, one queue per
  client, typed events (node | chat | packet | telemetry).
• Producers: data_store.update_node publishes node; mqtt.handle_log
  publishes packet (raw msg + mqtt_row_id), handle_telemetry publishes
  telemetry, handle_text publishes chat. Payloads are isolated snapshots.
• Caddy (Caddyfile.dev/.sample): proxy tuned for SSE (no buffering, long
  timeouts) on the /v1/events route.
• Tests: tests/test_broadcaster.py (fan-out, drop-oldest, snapshot
  isolation) and tests/test_mqtt_handlers.py (packet/telemetry/chat publish).

Frontend — shared live infrastructure

• LiveEventsProvider (components/LiveEvents.tsx): owns the single
  EventSource, coalesces node bursts into the RTK Query getNodes cache
  (~400 ms), pings chat, and resyncs on (re)connect. Exposed via context.
• useLiveEvent(type, handler) (hooks/useLiveEvent.ts): subscribe any
  component to an extra event type on the one shared connection.
• LivePill (components/LivePill.tsx): shared 3-state (live / paused / off)
  indicator; top bars unified across Logs, Chat, Nodes, Stats, Neighbors,
  Telemetry, and Traceroutes.

Bug Fix

• Map preview on nodes incorrectly displayed. Closes Bug(map): selecting node on nodes page renders map preview incorrectly #489

Per-page live features

• Logs: live packet feed over SSE, anchored to the top, pauses on scroll-away
  or packet selection, de-duped against the fetched archive.
• Telemetry: live telemetry stream with the same scroll/select freeze.
• Chat / Nodes / Stats / Neighbors / Traceroutes: live node + chat updates
  push straight into the cache; no manual refresh.

Live map

• New nodes appear live and node brightness encodes recency: a quantized
  dim (by last_seen age) drives circle opacity, so nodes brighten when
  re-heard and fade as they go quiet. Quantized into buckets so re-hears don't
  churn the source; a nodesDataSignature skip-guard avoids redundant setData
  (and the cluster-donut rebuild each one triggers) on no-op flushes.
• Animated cluster ratio ring (clusterDonutLayer.ts): donuts ease between
  real online/total ratios instead of snapping, keyed by position so tweens
  survive cluster-id churn, with a self-stopping repaint clock and
  prefers-reduced-motion support.
• Live cluster hover card (ClusterHoverCard.tsx): hovering a cluster shows
  node count, online ratio, role breakdown, and a last-heard leaderboard, all
  hydrated from the live node cache so it stays current while open.
• Groundwork: normalizeNodeId8 (+tests) and a reducedMotion helper.