Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/crosschain/amb/CrossChainEnabledAMB.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.7.0) (crosschain/amb/CrossChainEnabledAMB.sol)
+
+pragma solidity ^0.8.4;
+
+import "../CrossChainEnabled.sol";
+import "./LibAMB.sol";
+
+/**
+ * @dev https://docs.tokenbridge.net/amb-bridge/about-amb-bridge[AMB]
+ * specialization or the {CrossChainEnabled} abstraction.
+ *
+ * As of february 2020, AMB bridges are available between the following chains:
+ *
+ * - https://docs.tokenbridge.net/eth-xdai-amb-bridge/about-the-eth-xdai-amb[ETH ⇌ xDai]
+ * - https://docs.tokenbridge.net/eth-qdai-bridge/about-the-eth-qdai-amb[ETH ⇌ qDai]
+ * - https://docs.tokenbridge.net/eth-etc-amb-bridge/about-the-eth-etc-amb[ETH ⇌ ETC]
+ * - https://docs.tokenbridge.net/eth-bsc-amb/about-the-eth-bsc-amb[ETH ⇌ BSC]
+ * - https://docs.tokenbridge.net/eth-poa-amb-bridge/about-the-eth-poa-amb[ETH ⇌ POA]
+ * - https://docs.tokenbridge.net/bsc-xdai-amb/about-the-bsc-xdai-amb[BSC ⇌ xDai]
+ * - https://docs.tokenbridge.net/poa-xdai-amb/about-the-poa-xdai-amb[POA ⇌ xDai]
+ * - https://docs.tokenbridge.net/rinkeby-xdai-amb-bridge/about-the-rinkeby-xdai-amb[Rinkeby ⇌ xDai]
+ * - https://docs.tokenbridge.net/kovan-sokol-amb-bridge/about-the-kovan-sokol-amb[Kovan ⇌ Sokol]
+ *
+ * _Available since v4.6._
+ */
+contract CrossChainEnabledAMB is CrossChainEnabled {
+    /// @custom:oz-upgrades-unsafe-allow state-variable-immutable
+    address private immutable _bridge;
+
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address bridge) {
+        _bridge = bridge;
+    }
+
+    /**
+     * @dev see {CrossChainEnabled-_isCrossChain}
+     */
+    function _isCrossChain() internal view virtual override returns (bool) {
+        return LibAMB.isCrossChain(_bridge);
+    }
+
+    /**
+     * @dev see {CrossChainEnabled-_crossChainSender}
+     */
+    function _crossChainSender() internal view virtual override onlyCrossChain returns (address) {
+        return LibAMB.crossChainSender(_bridge);
+    }
+}

backend/lib/openzeppelin-contracts/contracts/crosschain/amb/LibAMB.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.7.0) (crosschain/amb/LibAMB.sol)
+
+pragma solidity ^0.8.4;
+
+import {IAMB as AMB_Bridge} from "../../vendor/amb/IAMB.sol";
+import "../errors.sol";
+
+/**
+ * @dev Primitives for cross-chain aware contracts using the
+ * https://docs.tokenbridge.net/amb-bridge/about-amb-bridge[AMB]
+ * family of bridges.
+ */
+library LibAMB {
+    /**
+     * @dev Returns whether the current function call is the result of a
+     * cross-chain message relayed by `bridge`.
+     */
+    function isCrossChain(address bridge) internal view returns (bool) {
+        return msg.sender == bridge;
+    }
+
+    /**
+     * @dev Returns the address of the sender that triggered the current
+     * cross-chain message through `bridge`.
+     *
+     * NOTE: {isCrossChain} should be checked before trying to recover the
+     * sender, as it will revert with `NotCrossChainCall` if the current
+     * function call is not the result of a cross-chain message.
+     */
+    function crossChainSender(address bridge) internal view returns (address) {
+        if (!isCrossChain(bridge)) revert NotCrossChainCall();
+        return AMB_Bridge(bridge).messageSender();
+    }
+}