v0.5.0: Backup bugfixes + monitoring page with metrics store

- Fix "Helyi mentés" showing "–" after controller restart by synthesizing
  LastBackup from snapshot history and LastDBDump from dump files on disk
- New monitoring page (/monitoring) with system info, metrics charts, and
  container resource overview
- SQLite metrics store (modernc.org/sqlite, pure Go, no CGO) with 60s
  collection interval and 30-day auto-prune
- REST API endpoints: /api/metrics/system, /api/metrics/containers/summary,
  /api/metrics/containers/{name}, /api/metrics/sysinfo
- Chart.js 4.4.7 embedded locally for offline environments
- System info provider reads hostname, OS, kernel, CPU, uptime from /proc
- Docker compose updated with /etc/os-release host mount

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

controller/internal/web/templates/layout.html

@@ -17,6 +17,7 @@
             <li><a href="/" class="{{if eq .Page "dashboard"}}active{{end}}">Vezérlőpult</a></li>
             <li><a href="/stacks" class="{{if eq .Page "stacks"}}active{{end}}">Alkalmazások</a></li>
             <li><a href="/backups" class="{{if eq .Page "backups"}}active{{end}}">Biztonsági mentés</a></li>
+            <li><a href="/monitoring" class="{{if eq .Page "monitoring"}}active{{end}}">Rendszermonitor</a></li>
         </ul>
         <div class="sidebar-footer">
             <span class="version">v{{.Version}}</span>