feat(backend): implement step 6 — AiService with OpenAI RestClient integration

backend/src/main/kotlin/com/condado/newsletter/config/AppConfig.kt

@@ -0,0 +1,15 @@
+package com.condado.newsletter.config
+
+import org.springframework.context.annotation.Bean
+import org.springframework.context.annotation.Configuration
+import org.springframework.web.client.RestClient
+
+/**
+ * Application-wide bean configuration.
+ */
+@Configuration
+class AppConfig {
+
+    @Bean
+    fun restClient(): RestClient = RestClient.create()
+}