Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC20/extensions/ERC20Pausable.sol

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (token/ERC20/extensions/ERC20Pausable.sol)
+
+pragma solidity ^0.8.0;
+
+import "../ERC20.sol";
+import "../../../security/Pausable.sol";
+
+/**
+ * @dev ERC20 token with pausable token transfers, minting and burning.
+ *
+ * Useful for scenarios such as preventing trades until the end of an evaluation
+ * period, or having an emergency switch for freezing all token transfers in the
+ * event of a large bug.
+ *
+ * IMPORTANT: This contract does not include public pause and unpause functions. In
+ * addition to inheriting this contract, you must define both functions, invoking the
+ * {Pausable-_pause} and {Pausable-_unpause} internal functions, with appropriate
+ * access control, e.g. using {AccessControl} or {Ownable}. Not doing so will
+ * make the contract unpausable.
+ */
+abstract contract ERC20Pausable is ERC20, Pausable {
+    /**
+     * @dev See {ERC20-_beforeTokenTransfer}.
+     *
+     * Requirements:
+     *
+     * - the contract must not be paused.
+     */
+    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual override {
+        super._beforeTokenTransfer(from, to, amount);
+
+        require(!paused(), "ERC20Pausable: token transfer while paused");
+    }
+}