feat: backup system for portal DB and WLED configs

Portal DB:
- Download current DB button
- Manual backup creation
- Auto-backup daily at 02:00 (keeps last 14)
- Download/delete individual backups

WLED Config:
- Backup all devices at once
- Backup individual device
- Download config as JSON
- Restore config to any device
- Delete old backups

New sidebar menu item 'Backup' with dedicated page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/base.html

@@ -42,6 +42,11 @@
                         <i class="bi bi-palette"></i> Farb-Regeln
                     </a>
                 </li>
+                <li class="nav-item">
+                    <a class="nav-link text-white {% if page == 'backup' %}active{% endif %}" href="/backup">
+                        <i class="bi bi-download"></i> Backup
+                    </a>
+                </li>
                 <li class="nav-item">
                     <a class="nav-link text-white {% if page == 'settings' %}active{% endif %}" href="/settings">
                         <i class="bi bi-gear"></i> Einstellungen