v0.55.0: Phase 3 — auto off-drive Tier 2 (rootfs-headroom guard)

Tier 2 rsync-mirrors each HDD app's recovery unit + appdata to a DIFFERENT physical
disk (the only off-drive protection bind-mounted userdata can get; PBS can't reach it).
Auto-enabled, auto-target: prefer another registered drive (different physical disk via
system.SamePhysicalDevice), else the internal SSD for SMALL units only — with a
size-aware headroom guard that REFUSES rather than fill the ~8G guest rootfs, recording
an honest "needs 2nd HDD" status. Status persisted via the surviving CrossDriveBackup;
"2. mentés" UI card now populated. Daily tier2-backup job + POST /api/backup/tier2.

- backup/tier2.go (engine+selection+headroom), tier2_test.go (headroom arithmetic)
- system.SamePhysicalDevice (linux Stat_t.Dev + stub)
- handlers.go Tier2 UI population + tier2DestLabel; backups.html honest no-target reason
- fixed stale TestBackupCopiesOnPath (old felhom-data layout -> in-guest layout)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/backup/backup.go

@@ -28,6 +28,9 @@ type Manager struct {
 	systemDataPath string // fallback drive for SSD-only apps
 	version        string // controller version, stamped into recovery-unit manifests
 
+	// tier2Notify, if set, is called after each Tier 2 copy (success: err==nil) for notifications.
+	tier2Notify func(stackName, destLabel string, dur time.Duration, err error)
+
 	mu         sync.Mutex
 	lastDBDump *DBDumpStatus
 	running    bool