feat(backend): add distribution queue

refs #13

backend/contracts/DistributionQueue.sol

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.24;
+
+enum DistributionStatus {
+    DeviceDelivered,
+    Approved,
+    Rejected,
+    Paid
+}
+
+struct Distribution {
+    uint256 timestamp;
+    address distributor;
+    DistributionStatus status;
+}
+
+contract DistributionQueue {
+    address public owner;
+    address public attestationHandler;
+    Distribution[] public queue;
+
+    event OwnerUpdated(address owner);
+    event AttestationHandlerUpdated(address attestationHandler);
+    event DistributionAdded(Distribution distribution);
+    event DistributionStatusUpdated(Distribution distribution);
+
+    error OnlyOwner();
+    error OnlyAttestationHandler();
+
+    modifier onlyOwner() {
+        if (msg.sender != owner) {
+            revert OnlyOwner();
+        }
+        _;
+    }
+
+    modifier onlyAttestationHandler() {
+        if (msg.sender != owner) {
+            revert OnlyAttestationHandler();
+        }
+        _;
+    }
+
+    constructor(address _attestationHandler) {
+        owner = msg.sender;
+        attestationHandler = _attestationHandler;
+    }
+
+    function updateOwner(address _owner) public onlyOwner() {
+        owner = _owner;
+        emit OwnerUpdated(_owner);
+    }
+
+    function updateAttestationHandler(address _attestationHandler) public onlyAttestationHandler() {
+        attestationHandler = _attestationHandler;
+        emit AttestationHandlerUpdated(_attestationHandler);
+    }
+
+    function addDistribution() public {
+        Distribution memory distribution = Distribution(
+            block.timestamp,
+            msg.sender,
+            DistributionStatus.DeviceDelivered
+        );
+        queue.push(distribution);
+        emit DistributionAdded(distribution);
+    }
+
+    function updateDistributionStatus(uint256 queueIndex, DistributionStatus _distributionStatus) public onlyAttestationHandler() {
+        Distribution memory distribution = queue[queueIndex];
+        distribution.status = _distributionStatus;
+        emit DistributionStatusUpdated(distribution);
+    }
+
+    function getQueueCount() public view returns (uint256) {
+        return queue.length;
+    }
+}