Add database backup/restore and WebDAV backup features

Developer tab now includes:
- Database Backup & Restore section with download/upload JSON backup
- WebDAV Backup section to upload backups to Nextcloud/WebDAV servers
  with configurable server URL, username, and password

Features:
- Full JSON export of entries, settings, logos, users, license
- Import with data replacement and transaction safety
- WebDAV upload via cURL with PUT request
- Automatic timestamped filenames (ipmanager-YYYY-MM-DD_HH-mm-ss.json)
- Settings persistence for WebDAV credentials

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

database/schema.sql

@@ -171,7 +171,10 @@ INSERT INTO geofeed_settings (setting_key, setting_value) VALUES
 ('whitelabel_company_name', ''),
 ('whitelabel_icon_url', ''),
 ('whitelabel_favicon_url', ''),
-('whitelabel_default_import_url', '')
+('whitelabel_default_import_url', ''),
+('webdav_server_url', ''),
+('webdav_username', ''),
+('webdav_password', '')
 ON DUPLICATE KEY UPDATE setting_key = setting_key;
 
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