Switch from maven to gradle.

backend/build.gradle.kts

@@ -0,0 +1,56 @@
+plugins {
+    kotlin("jvm") version "2.2.20"
+    kotlin("plugin.spring") version "2.2.20"
+    id("io.spring.dependency-management") version "1.1.7"
+}
+
+group = "org.codiki"
+version = "0.0.1-SNAPSHOT"
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(21)
+    }
+}
+
+repositories {
+    mavenCentral()
+}
+
+subprojects {
+    apply(plugin = "java")
+    apply(plugin = "org.jetbrains.kotlin.jvm")
+
+    repositories {
+        mavenCentral()
+    }
+
+    dependencies {
+        implementation(platform("org.springframework.boot:spring-boot-dependencies:3.5.5"))
+        compileOnly("org.projectlombok:lombok:1.18.40")
+        annotationProcessor("org.projectlombok:lombok:1.18.40")
+        testImplementation("org.assertj:assertj-core:3.27.4")
+        testImplementation("org.junit.jupiter:junit-jupiter-api")
+        testImplementation("org.junit.jupiter:junit-jupiter-params")
+    }
+
+    tasks.withType<Test> {
+        useJUnitPlatform()
+    }
+}
+
+dependencyManagement {
+    imports {
+        mavenBom("org.springframework.boot:spring-boot-dependencies:3.5.5")
+    }
+}
+
+kotlin {
+    compilerOptions {
+        freeCompilerArgs.addAll("-Xjsr305=strict")
+    }
+}
+
+tasks.withType<Test> {
+    useJUnitPlatform()
+}