Build: enable ktlint+detekt, format sources, add core unit tests

libs/test-support/build.gradle.kts

@@ -3,11 +3,14 @@ plugins {
 }
 
 dependencies {
-    implementation(platform(libs.springBootBom))
+    api(platform(libs.springBootBom))
 
     // Shared integration-test infrastructure. Consumers add this module as testImplementation.
     api(libs.springBootTestcontainers)
     api(libs.testcontainersPostgresql)
     api(libs.testcontainersJunitJupiter)
     api(libs.springBootStarterTest)
+    api(libs.mockk)
+    // Gradle 9 требует launcher JUnit Platform на тестовом classpath явно.
+    api(libs.junitPlatformLauncher)
 }