Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC20/extensions/ERC20Capped.sol

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (token/ERC20/extensions/ERC20Capped.sol)
+
+pragma solidity ^0.8.0;
+
+import "../ERC20.sol";
+
+/**
+ * @dev Extension of {ERC20} that adds a cap to the supply of tokens.
+ */
+abstract contract ERC20Capped is ERC20 {
+    uint256 private immutable _cap;
+
+    /**
+     * @dev Sets the value of the `cap`. This value is immutable, it can only be
+     * set once during construction.
+     */
+    constructor(uint256 cap_) {
+        require(cap_ > 0, "ERC20Capped: cap is 0");
+        _cap = cap_;
+    }
+
+    /**
+     * @dev Returns the cap on the token's total supply.
+     */
+    function cap() public view virtual returns (uint256) {
+        return _cap;
+    }
+
+    /**
+     * @dev See {ERC20-_mint}.
+     */
+    function _mint(address account, uint256 amount) internal virtual override {
+        require(ERC20.totalSupply() + amount <= cap(), "ERC20Capped: cap exceeded");
+        super._mint(account, amount);
+    }
+}