Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/interfaces/IERC2309.sol

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.8.0) (interfaces/IERC2309.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev ERC-2309: ERC-721 Consecutive Transfer Extension.
+ *
+ * _Available since v4.8._
+ */
+interface IERC2309 {
+    /**
+     * @dev Emitted when the tokens from `fromTokenId` to `toTokenId` are transferred from `fromAddress` to `toAddress`.
+     */
+    event ConsecutiveTransfer(
+        uint256 indexed fromTokenId,
+        uint256 toTokenId,
+        address indexed fromAddress,
+        address indexed toAddress
+    );
+}