Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/Timers.sol

@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (utils/Timers.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Tooling for timepoints, timers and delays
+ *
+ * CAUTION: This file is deprecated as of 4.9 and will be removed in the next major release.
+ */
+library Timers {
+    struct Timestamp {
+        uint64 _deadline;
+    }
+
+    function getDeadline(Timestamp memory timer) internal pure returns (uint64) {
+        return timer._deadline;
+    }
+
+    function setDeadline(Timestamp storage timer, uint64 timestamp) internal {
+        timer._deadline = timestamp;
+    }
+
+    function reset(Timestamp storage timer) internal {
+        timer._deadline = 0;
+    }
+
+    function isUnset(Timestamp memory timer) internal pure returns (bool) {
+        return timer._deadline == 0;
+    }
+
+    function isStarted(Timestamp memory timer) internal pure returns (bool) {
+        return timer._deadline > 0;
+    }
+
+    function isPending(Timestamp memory timer) internal view returns (bool) {
+        return timer._deadline > block.timestamp;
+    }
+
+    function isExpired(Timestamp memory timer) internal view returns (bool) {
+        return isStarted(timer) && timer._deadline <= block.timestamp;
+    }
+
+    struct BlockNumber {
+        uint64 _deadline;
+    }
+
+    function getDeadline(BlockNumber memory timer) internal pure returns (uint64) {
+        return timer._deadline;
+    }
+
+    function setDeadline(BlockNumber storage timer, uint64 timestamp) internal {
+        timer._deadline = timestamp;
+    }
+
+    function reset(BlockNumber storage timer) internal {
+        timer._deadline = 0;
+    }
+
+    function isUnset(BlockNumber memory timer) internal pure returns (bool) {
+        return timer._deadline == 0;
+    }
+
+    function isStarted(BlockNumber memory timer) internal pure returns (bool) {
+        return timer._deadline > 0;
+    }
+
+    function isPending(BlockNumber memory timer) internal view returns (bool) {
+        return timer._deadline > block.number;
+    }
+
+    function isExpired(BlockNumber memory timer) internal view returns (bool) {
+        return isStarted(timer) && timer._deadline <= block.number;
+    }
+}