Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/interfaces/IERC4906.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (interfaces/IERC4906.sol)
+
+pragma solidity ^0.8.0;
+
+import "./IERC165.sol";
+import "./IERC721.sol";
+
+/// @title EIP-721 Metadata Update Extension
+interface IERC4906 is IERC165, IERC721 {
+    /// @dev This event emits when the metadata of a token is changed.
+    /// So that the third-party platforms such as NFT market could
+    /// timely update the images and related attributes of the NFT.
+    event MetadataUpdate(uint256 _tokenId);
+
+    /// @dev This event emits when the metadata of a range of tokens is changed.
+    /// So that the third-party platforms such as NFT market could
+    /// timely update the images and related attributes of the NFTs.
+    event BatchMetadataUpdate(uint256 _fromTokenId, uint256 _toTokenId);
+}