feat: add 7 new features — exec shell, SSE events, compose import, docker resources, resource limits, email notifications, auto-updates

- Container exec/shell via WebSocket + xterm.js (subtle >_ button on cards)
- Live dashboard updates via SSE (resource alerts, health changes, update notices)
- Docker Compose import with YAML parsing, preview, and dependency-ordered deploy
- Volume & network management modal with disk usage overview
- CPU/memory resource limits on deploy and live update
- Email SMTP notifications (nodemailer) alongside Discord/Telegram/ntfy
- Scheduled auto-update scheduler with maintenance windows (daily/weekly/monthly)

New deps: ws, js-yaml, nodemailer

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

dashcaddy-api/package.json

@@ -19,14 +19,17 @@
     "express": "^4.22.1",
     "express-rate-limit": "^7.5.1",
     "helmet": "^8.1.0",
+    "js-yaml": "^4.1.1",
     "jsonwebtoken": "^9.0.2",
     "lru-cache": "^10.4.3",
+    "nodemailer": "^8.0.4",
     "otplib": "^12.0.1",
     "png-to-ico": "^2.1.8",
     "proper-lockfile": "^4.1.2",
     "qrcode": "^1.5.3",
     "sharp": "^0.33.5",
-    "validator": "^13.11.0"
+    "validator": "^13.11.0",
+    "ws": "^8.20.0"
   },
   "devDependencies": {
     "eslint": "^8.57.1",