v0.9.0: Storage paths registry, per-app HDD_PATH resolution, storage management UI

- Fix backup toggles not appearing (read each app's own HDD_PATH from app.yaml)
- Storage paths registry in settings.json with auto-discovery from deployed apps
- Settings page "Adattárolók" section with disk usage, add/remove/default/schedulable
- Deploy page path field as dropdown of registered storage paths
- Health check storage monitoring (mount point, disk usage alerts)
- Mount-point validation utilities (Linux syscall + cross-platform stubs)
- Controller docker-compose mount changed to /mnt:/mnt:rw for multi-storage

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

controller/internal/system/mounts_linux.go

@@ -0,0 +1,92 @@
+//go:build linux
+
+package system
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"syscall"
+)
+
+// IsMountPoint checks if a path is on a different device than its parent.
+// Returns true if the path is a mount point (different device ID from parent).
+func IsMountPoint(path string) bool {
+	var pathStat, parentStat syscall.Stat_t
+	if err := syscall.Stat(path, &pathStat); err != nil {
+		return false
+	}
+	parent := filepath.Dir(path)
+	if err := syscall.Stat(parent, &parentStat); err != nil {
+		return false
+	}
+	return pathStat.Dev != parentStat.Dev
+}
+
+// IsWritable checks if the given path is writable by attempting to create+remove a temp file.
+func IsWritable(path string) bool {
+	testFile := filepath.Join(path, ".felhom-write-test")
+	f, err := os.Create(testFile)
+	if err != nil {
+		return false
+	}
+	f.Close()
+	os.Remove(testFile)
+	return true
+}
+
+// PathsOverlap returns true if one path is a parent or child of the other.
+func PathsOverlap(a, b string) bool {
+	a = filepath.Clean(a)
+	b = filepath.Clean(b)
+	if a == b {
+		return true
+	}
+	aSep := a + string(os.PathSeparator)
+	bSep := b + string(os.PathSeparator)
+	return strings.HasPrefix(aSep, bSep) || strings.HasPrefix(bSep, aSep)
+}
+
+// DiskUsageInfo holds disk usage statistics for a path.
+type DiskUsageInfo struct {
+	TotalGB     float64
+	UsedGB      float64
+	AvailGB     float64
+	UsedPercent float64
+	TotalHuman  string
+	UsedHuman   string
+}
+
+// GetDiskUsage returns disk usage info for a path, or nil on error.
+func GetDiskUsage(path string) *DiskUsageInfo {
+	var stat syscall.Statfs_t
+	if err := syscall.Statfs(path, &stat); err != nil {
+		return nil
+	}
+
+	bsize := uint64(stat.Bsize)
+	total := stat.Blocks * bsize
+	avail := stat.Bavail * bsize
+	used := total - (stat.Bfree * bsize)
+
+	const gb = 1024 * 1024 * 1024
+	info := &DiskUsageInfo{
+		TotalGB: float64(total) / float64(gb),
+		UsedGB:  float64(used) / float64(gb),
+		AvailGB: float64(avail) / float64(gb),
+	}
+	if total > 0 {
+		info.UsedPercent = float64(used) / float64(total) * 100
+	}
+	info.TotalHuman = formatGB(info.TotalGB)
+	info.UsedHuman = formatGB(info.UsedGB)
+	return info
+}
+
+func formatGB(gb float64) string {
+	if gb >= 1000 {
+		return fmt.Sprintf("%.1f TB", gb/1024)
+	}
+	return fmt.Sprintf("%.1f GB", gb)
+}