feat(backend): set sponsorship cost

refs #10

backend/contracts/SponsorshipProgram.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.24;
+
+/**
+ * A system for sponsoring the education of one individual child.
+ */
+contract SponsorshipProgram {
+    address public owner;
+    uint256 public sponsorshipCost;
+
+    event SponsorshipCostUpdated(uint256 amount);
+
+    error OnlyOwner();
+
+    modifier onlyOwner() {
+        if (msg.sender != owner) {
+            revert OnlyOwner();
+        }
+        _;
+    }
+
+    constructor(uint256 _sponsorshipCost) {
+        owner = msg.sender;
+        sponsorshipCost = _sponsorshipCost;
+    }
+
+    function updateSponsorshipCost(uint256 amount) public onlyOwner {
+        sponsorshipCost = amount;
+        emit SponsorshipCostUpdated(amount);
+    }
+}

backend/test/SponsorshipProgram.ts

@@ -0,0 +1,38 @@
+import {
+  loadFixture,
+} from "@nomicfoundation/hardhat-toolbox/network-helpers";
+import { expect } from "chai";
+import hre from "hardhat";
+
+describe("SponsorshipProgram", function () {
+  // We define a fixture to reuse the same setup in every test.
+  // We use loadFixture to run this setup once, snapshot that state,
+  // and reset Hardhat Network to that snapshot in every test.
+  async function deployFixture() {
+    // Contracts are deployed using the first signer/account by default
+    const [owner, otherAccount] = await hre.ethers.getSigners();
+
+    const sponsorshipCost = hre.ethers.parseUnits("0.02");
+
+    const SponsorshipProgram = await hre.ethers.getContractFactory("SponsorshipProgram");
+    const sponsorshipProgram = await SponsorshipProgram.deploy(sponsorshipCost);
+
+    return { sponsorshipProgram, owner, otherAccount };
+  }
+
+  describe("Deployment", function () {
+    it("Should set the right sponshorship cost", async function () {
+      const { sponsorshipProgram } = await loadFixture(deployFixture);
+
+      const expectedValue = hre.ethers.parseUnits("0.02");
+      console.log("expectedValue:", expectedValue);
+      expect(await sponsorshipProgram.sponsorshipCost()).to.equal(expectedValue);
+    });
+
+    it("Should set the right owner", async function () {
+      const { sponsorshipProgram, owner } = await loadFixture(deployFixture);
+
+      expect(await sponsorshipProgram.owner()).to.equal(owner.address);
+    });
+  });
+});