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/server.js

@@ -52,6 +52,11 @@ process.on('uncaughtException', (error) => {
         environment: process.env.NODE_ENV || 'production'
       });
 
+      // Attach WebSocket exec handler
+      const attachExecWS = require('./routes/exec');
+      attachExecWS(server, log);
+      log.info('server', 'WebSocket exec handler attached');
+
       // Start feature modules
       const resourceMonitor = require('./resource-monitor');
       const backupManager = require('./backup-manager');