v0.9.0: in-app auto-updater, ship toggle split, stable entity IDs, performance fixes

New features:
- In-app auto-updater with confirmation dialog, manual download fallback,
  restart polling, and protected file safety net
- Ship layers split into 4 independent toggles (Military/Carriers, Cargo/Tankers,
  Civilian, Cruise/Passenger) with per-category counts
- Stable entity IDs using MMSI/callsign instead of volatile array indices
- Dismissible threat alert bubbles (session-scoped, survives data refresh)

Performance:
- GDELT title fetching is now non-blocking (background enrichment)
- Removed duplicate startup fetch jobs
- Docker healthcheck start_period 15s → 90s

Bug fixes:
- Removed fake intelligence assessment generator (OSINT-only policy)
- Fixed carrier tracker GDELT 429/TypeError crash
- Fixed ETag collision (full payload hash)
- Added concurrent /api/refresh guard

Contributors: @imqdcr (ship split + stable IDs), @csysp (dismissible alerts, PR #48)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Former-commit-id: a2c4c67da54345393f70a9b33b52e7e4fd6c049f

frontend/src/components/map/mapConstants.ts

@@ -0,0 +1,5 @@
+// Shared map constants
+// Extracted from MaplibreViewer.tsx
+
+// Empty GeoJSON constant — avoids recreating empty objects on every render
+export const EMPTY_FC: GeoJSON.FeatureCollection = { type: 'FeatureCollection', features: [] };