Add IdempotencyPlugin for handling Idempotency-Key headers in Ktor applications

backend/services/masterdata/masterdata-api/src/main/kotlin/at/mocode/masterdata/api/plugins/IdempotencyPlugin.kt

@@ -0,0 +1,26 @@
+package at.mocode.masterdata.api.plugins
+
+import io.ktor.server.application.*
+import io.ktor.util.*
+import java.time.Duration
+
+/**
+ * Minimaler Ktor-Plugin-Skeleton für Idempotency-Key Verarbeitung.
+ * Aktuell: Stellt den Header `Idempotency-Key` als Call-Attribut bereit.
+ * Erweiterung (Cache/Short-Circuit) folgt im nächsten Schritt.
+ */
+object IdempotencyPlugin {
+  @JvmInline
+  value class Configuration(val ttl: Duration = Duration.ofMinutes(5))
+
+  val IdempotencyKeyAttr: AttributeKey<String> = AttributeKey("IdempotencyKey")
+
+  fun install(application: Application, configuration: Configuration = Configuration()) {
+    application.intercept(ApplicationCallPipeline.Plugins) {
+      val key = call.request.headers["Idempotency-Key"]?.trim()
+      if (!key.isNullOrBlank()) {
+        call.attributes.put(IdempotencyKeyAttr, key)
+      }
+    }
+  }
+}