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

backend/config/news_feeds.json

@@ -0,0 +1,12 @@
+{
+  "feeds": [
+    { "name": "NPR", "url": "https://feeds.npr.org/1004/rss.xml", "weight": 4 },
+    { "name": "BBC", "url": "http://feeds.bbci.co.uk/news/world/rss.xml", "weight": 3 },
+    { "name": "AlJazeera", "url": "https://www.aljazeera.com/xml/rss/all.xml", "weight": 2 },
+    { "name": "NYT", "url": "https://rss.nytimes.com/services/xml/rss/nyt/World.xml", "weight": 1 },
+    { "name": "GDACS", "url": "https://www.gdacs.org/xml/rss.xml", "weight": 5 },
+    { "name": "NHK", "url": "https://www3.nhk.or.jp/nhkworld/rss/world.xml", "weight": 3 },
+    { "name": "CNA", "url": "https://www.channelnewsasia.com/rssfeed/8395986", "weight": 3 },
+    { "name": "Mercopress", "url": "https://en.mercopress.com/rss/", "weight": 3 }
+  ]
+}