Feature: Multi-email alerts and move resource between groups

- notificationService: Parse comma-separated emails from resource.email_to or global config; validate and send to each recipient
- API: Add PATCH /api/resources/:id/group endpoint to move resource between groups with validation and cache invalidation
- UI: Add group selector to edit modal; wire group change to PATCH endpoint; add help text for multi-email format

client/build/index.html

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-<!doctype html><html lang="en"><head><meta charset="utf-8"/><link rel="icon" href="/favicon.ico"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><meta name="description" content="SkyWatch - Infrastructure monitoring and alerting system"/><link rel="apple-touch-icon" href="/logo192.png"/><link rel="manifest" href="/manifest.json"/><title>SkyWatch</title><script defer="defer" src="/static/js/main.9a96e479.js"></script><link href="/static/css/main.2e6b09f5.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>
+<!doctype html><html lang="en"><head><meta charset="utf-8"/><link rel="icon" href="/favicon.ico"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><meta name="description" content="SkyWatch - Infrastructure monitoring and alerting system"/><link rel="apple-touch-icon" href="/logo192.png"/><link rel="manifest" href="/manifest.json"/><title>SkyWatch</title><script defer="defer" src="/static/js/main.b7b055d1.js"></script><link href="/static/css/main.2e6b09f5.css" rel="stylesheet"></head><body><noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div></body></html>