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/handlers.go

@@ -191,6 +191,12 @@ func (s *Server) appDetailHandler(w http.ResponseWriter, _ *http.Request, slug s
 	s.render(w, "app_info", data)
 }
 
+func (s *Server) monitoringHandler(w http.ResponseWriter, _ *http.Request) {
+	data := s.baseData("monitoring", "Rendszermonitor")
+	data["SystemInfo"] = system.GetInfo(s.cfg.Paths.HDDPath, s.cpuCollector)
+	s.render(w, "monitoring", data)
+}
+
 func (s *Server) backupsHandler(w http.ResponseWriter, _ *http.Request) {
 	data := s.baseData("backups", "Biztonsági mentés")