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/system/mounts_other.go

@@ -48,6 +48,9 @@ type DiskUsageInfo struct {
 // GetDiskUsage returns nil on non-Linux.
 func GetDiskUsage(_ string) *DiskUsageInfo { return nil }
 
+// SamePhysicalDevice always returns false on non-Linux (dev/testing only — Tier 2 runs on Linux).
+func SamePhysicalDevice(_, _ string) bool { return false }
+
 // FSInfo holds filesystem type, device, and disk model info.
 type FSInfo struct {
 	FSType string