Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/metatx/ERC2771Context.sol

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.7.0) (metatx/ERC2771Context.sol)
+
+pragma solidity ^0.8.9;
+
+import "../utils/Context.sol";
+
+/**
+ * @dev Context variant with ERC2771 support.
+ */
+abstract contract ERC2771Context is Context {
+    /// @custom:oz-upgrades-unsafe-allow state-variable-immutable
+    address private immutable _trustedForwarder;
+
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address trustedForwarder) {
+        _trustedForwarder = trustedForwarder;
+    }
+
+    function isTrustedForwarder(address forwarder) public view virtual returns (bool) {
+        return forwarder == _trustedForwarder;
+    }
+
+    function _msgSender() internal view virtual override returns (address sender) {
+        if (isTrustedForwarder(msg.sender)) {
+            // The assembly code is more direct than the Solidity version using `abi.decode`.
+            /// @solidity memory-safe-assembly
+            assembly {
+                sender := shr(96, calldataload(sub(calldatasize(), 20)))
+            }
+        } else {
+            return super._msgSender();
+        }
+    }
+
+    function _msgData() internal view virtual override returns (bytes calldata) {
+        if (isTrustedForwarder(msg.sender)) {
+            return msg.data[:msg.data.length - 20];
+        } else {
+            return super._msgData();
+        }
+    }
+}