Add backup_path config field to Nextcloud integration

Allows a Nextcloud subdirectory to be specified as the target for
stack-level backups run by the management backup service.

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

src/integrations/schema.js

@@ -14,7 +14,7 @@ export const INTEGRATIONS = {
   },
   nextcloud: {
     name: 'Nextcloud',
-    configFields: ['base_url', 'username'],
+    configFields: ['base_url', 'username', 'backup_path'],
     secretFields: ['password'],
   },
   azure: {