chore(build, docs): add security module and update infrastructure decisions

- Created `backend/infrastructure/security` module with standardized configurations for OAuth2, JWT validation, CORS, and role mapping.
- Updated ADRs to reflect resolved backend infrastructure decisions, including security standardization, persistence strategy, and Flyway schema location.
- Enabled integration of the `security` module into relevant projects (e.g., `ping-service` and `gateway`).

backend/infrastructure/security/build.gradle.kts

@@ -0,0 +1,40 @@
+plugins {
+    alias(libs.plugins.kotlinJvm)
+    alias(libs.plugins.kotlinSpring)
+    alias(libs.plugins.spring.boot)
+    alias(libs.plugins.spring.dependencyManagement)
+}
+
+// Library module: do not create an executable Spring Boot jar here.
+tasks.bootJar {
+    enabled = false
+}
+
+tasks.jar {
+    enabled = true
+}
+
+dependencies {
+    implementation(platform(projects.platform.platformBom))
+    implementation(projects.platform.platformDependencies)
+
+    // Spring Security & OAuth2
+    implementation(libs.spring.boot.starter.security)
+    implementation(libs.spring.boot.starter.oauth2.resource.server)
+    implementation(libs.spring.security.oauth2.jose)
+
+    // Web (for CORS config)
+    implementation(libs.spring.web)
+
+    // Utils
+    implementation(libs.slf4j.api)
+    implementation(libs.jackson.module.kotlin)
+
+    // Testing
+    testImplementation(projects.platform.platformTesting)
+    testImplementation(libs.spring.security.test)
+}
+
+tasks.test {
+    useJUnitPlatform()
+}