Add agent notifications + fix update script

- scheduler.js: check agents every 2 min — alert on offline/online
  transitions and CPU/memory/disk threshold breaches (90%)
- notificationService.js: sendAgentAlert() sends via email, webhook,
  ntfy, and saves to in-app notifications table
- package.json: pin better-sqlite3 to 9.6.0 (compatible with Node 20)
- update-server.sh: add React client build step; use npm install
  instead of rm -rf + npm ci for faster incremental updates

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

package.json

@@ -22,7 +22,7 @@
   "license": "MIT",
   "dependencies": {
     "axios": "^1.6.2",
-    "better-sqlite3": "^13.0.3",
+    "better-sqlite3": "9.6.0",
     "cors": "^2.8.5",
     "dotenv": "^16.3.1",
     "express": "^4.18.2",