v0.6.0: custom news feeds, data center map layer, performance hardening

New features:
- Custom RSS Feed Manager: add/remove/prioritize up to 20 news sources
  from the Settings panel with weight levels 1-5. Persists across restarts.
- Global Data Center Map Layer: 2,000+ DCs plotted worldwide with clustering,
  server-rack icons, and automatic internet outage cross-referencing.
- Imperative map rendering: high-volume layers bypass React reconciliation
  via direct setData() calls with debounced updates on dense layers.
- Enhanced /api/health with per-source freshness timestamps and counts.

Fixes:
- Data center coordinates fixed for 187 Southern Hemisphere entries
- Docker CORS_ORIGINS passthrough in docker-compose.yml
- Start scripts warn on Python 3.13+ compatibility
- Settings panel redesigned with tabbed UI (API Keys / News Feeds)

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

Former-commit-id: 950c308f04

docker-compose.yml

@@ -12,6 +12,8 @@ services:
       - OPENSKY_CLIENT_ID=${OPENSKY_CLIENT_ID}
       - OPENSKY_CLIENT_SECRET=${OPENSKY_CLIENT_SECRET}
       - LTA_ACCOUNT_KEY=${LTA_ACCOUNT_KEY}
+      # Override allowed CORS origins (comma-separated). Auto-detects LAN IPs if empty.
+      - CORS_ORIGINS=${CORS_ORIGINS:-}
     volumes:
       - backend_data:/app/data
     restart: unless-stopped