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()
+}

backend/src/main/kotlin/com/condado/newsletter/model/ParsedAiResponse.kt

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+package com.condado.newsletter.model
+
+/**
+ * The parsed result of an AI-generated email response.
+ */
+data class ParsedAiResponse(
+    val subject: String,
+    val body: String
+)

backend/src/main/kotlin/com/condado/newsletter/service/AiExceptions.kt

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+package com.condado.newsletter.service
+
+/** Thrown when the OpenAI API call fails. */
+class AiServiceException(message: String, cause: Throwable? = null) : RuntimeException(message, cause)
+
+/** Thrown when the AI response cannot be parsed into SUBJECT/BODY format. */
+class AiParseException(message: String) : RuntimeException(message)

backend/src/main/kotlin/com/condado/newsletter/service/AiService.kt

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+package com.condado.newsletter.service
+
+import com.condado.newsletter.model.ParsedAiResponse
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties
+import com.fasterxml.jackson.annotation.JsonProperty
+import org.slf4j.LoggerFactory
+import org.springframework.beans.factory.annotation.Value
+import org.springframework.stereotype.Service
+import org.springframework.web.client.RestClient
+
+/**
+ * Calls the OpenAI Chat Completions API to generate email content.
+ * Returns a [ParsedAiResponse] with the extracted subject and body.
+ */
+@Service
+class AiService(
+    private val restClient: RestClient,
+    @Value("\${openai.api-key}") private val apiKey: String,
+    @Value("\${openai.model:gpt-4o}") private val model: String
+) {
+    private val log = LoggerFactory.getLogger(javaClass)
+
+    /**
+     * Sends [prompt] to the OpenAI API and returns the parsed subject + body.
+     * @throws [AiServiceException] on API errors.
+     * @throws [AiParseException] if the response format is unexpected.
+     */
+    fun generate(prompt: String): ParsedAiResponse {
+        val rawText = try {
+            val request = ChatRequest(
+                model = model,
+                messages = listOf(ChatMessage(role = "user", content = prompt))
+            )
+            val json = restClient.post()
+                .uri("https://api.openai.com/v1/chat/completions")
+                .header("Authorization", "Bearer $apiKey")
+                .body(request)
+                .retrieve()
+                .body(String::class.java)
+                ?: throw AiServiceException("OpenAI returned an empty response")
+
+            extractContent(json)
+        } catch (e: AiServiceException) {
+            throw e
+        } catch (e: Exception) {
+            log.error("OpenAI API call failed: ${e.message}", e)
+            throw AiServiceException("OpenAI API call failed: ${e.message}", e)
+        }
+
+        return parseResponse(rawText)
+    }
+
+    /**
+     * Parses the raw AI output into a [ParsedAiResponse].
+     * Expected format:
+     * ```
+     * SUBJECT: <subject line>
+     * BODY:
+     * <email body>
+     * ```
+     */
+    fun parseResponse(raw: String): ParsedAiResponse {
+        val lines = raw.trim().lines()
+
+        val subjectLine = lines.firstOrNull { it.startsWith("SUBJECT:", ignoreCase = true) }
+            ?: throw AiParseException("AI response missing SUBJECT line. Raw response: $raw")
+
+        val bodyStart = lines.indexOfFirst { it.trim().equals("BODY:", ignoreCase = true) }
+        if (bodyStart == -1) throw AiParseException("AI response missing BODY: section. Raw response: $raw")
+
+        val subject = subjectLine.replaceFirst(Regex("^SUBJECT:\\s*", RegexOption.IGNORE_CASE), "").trim()
+        val body = lines.drop(bodyStart + 1).joinToString("\n").trim()
+
+        return ParsedAiResponse(subject = subject, body = body)
+    }
+
+    // ── internal JSON helpers ────────────────────────────────────────────────
+
+    private fun extractContent(json: String): String {
+        val mapper = com.fasterxml.jackson.databind.ObjectMapper()
+        val tree = mapper.readTree(json)
+        return tree["choices"]?.firstOrNull()?.get("message")?.get("content")?.asText()
+            ?.replace("\\n", "\n")
+            ?: throw AiServiceException("Unexpected OpenAI response structure: $json")
+    }
+
+    @JsonIgnoreProperties(ignoreUnknown = true)
+    data class ChatRequest(val model: String, val messages: List<ChatMessage>)
+
+    @JsonIgnoreProperties(ignoreUnknown = true)
+    data class ChatMessage(val role: String, val content: String)
+}