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/web/templates/backups.html

@@ -358,8 +358,10 @@
                         </div>
                     {{else}}
                         <span class="layer-auto-ok">✓ 1. mentés auto</span>
-                        <span class="layer-unconfigured">⚠ Nincs 2. másolat</span>
-                        <a href="/stacks/{{.StackName}}/deploy" class="btn btn-xs">Beállítás →</a>
+                        <span class="layer-unconfigured">⚠ Nincs 2. (off-drive) másolat</span>
+                        {{if .Tier2LastError}}
+                        <span class="layer-reason" style="opacity:.85" title="A 2. mentés automatikus — külön beállítás nem kell">{{.Tier2LastError}}</span>
+                        {{end}}
                     {{end}}
                 </div>
                 <!-- Tier 3: Remote backup (future) -->