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/metrics/sysinfo_other.go

@@ -0,0 +1,19 @@
+//go:build !linux
+
+package metrics
+
+import (
+	"os"
+	"runtime"
+)
+
+// GetStaticInfo returns placeholder system info on non-Linux platforms.
+func GetStaticInfo() StaticSystemInfo {
+	hostname, _ := os.Hostname()
+	return StaticSystemInfo{
+		Hostname:     hostname,
+		OS:           runtime.GOOS,
+		Architecture: runtime.GOARCH,
+		CPUCores:     runtime.NumCPU(),
+	}
+}