Add manual backup trigger endpoint

POST /api/v1/system/auto-backup/trigger creates a backup on demand
and returns the created file info (filename, size, timestamp).

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

server/src/wled_controller/core/backup/auto_backup.py

@@ -211,6 +211,14 @@ class AutoBackupEngine:
             raise ValueError("Invalid filename")
         return target
 
+    async def trigger_backup(self) -> dict:
+        """Manually trigger a backup and prune old ones. Returns the created backup info."""
+        await self._perform_backup()
+        self._prune_old_backups()
+        # Return the most recent backup entry
+        backups = self.list_backups()
+        return backups[0] if backups else {}
+
     def delete_backup(self, filename: str) -> None:
         target = self._safe_backup_path(filename)
         if not target.exists():