fix: backup validation display and debug logging

- Add 4-branch template guard to handle zero-value DumpValidation
  (shows "–" instead of misleading "Hiba" when validation never ran)
- Add debug logging to ValidateDump for all code paths
- Add cross-check re-validation in RefreshCache to heal stale
  lastDBDump validation state from disk

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

controller/internal/web/templates/backups.html

@@ -136,8 +136,10 @@
                                 <span class="validation-badge validation-na">–</span>
                             {{else if .Validation.Valid}}
                                 <span class="validation-badge validation-ok">{{.Validation.TableCount}} tábla</span>
-                            {{else}}
+                            {{else if .Validation.Error}}
                                 <span class="validation-badge validation-fail" title="{{.Validation.Error}}">Hiba</span>
+                            {{else}}
+                                <span class="validation-badge validation-na" title="Az érvényesítés nem futott le">–</span>
                             {{end}}
                         </td>
                         <td>