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/recovery_unit.go

@@ -53,6 +53,11 @@ func (m *Manager) SetVersion(v string) {
 	m.mu.Unlock()
 }
 
+// SetTier2Notifier wires the notification callback invoked after each Tier 2 copy.
+func (m *Manager) SetTier2Notifier(fn func(stackName, destLabel string, dur time.Duration, err error)) {
+	m.tier2Notify = fn
+}
+
 // CaptureRecoveryUnit writes/refreshes an app's secret-free recovery unit: it captures the
 // compose + metadata + a secret-stripped app.yaml into compose/, enumerates the DB/volume dumps
 // already present, and writes manifest.json. It NEVER writes a secret value or the Docker image.