feat(backend): implement step 7 — EmailSenderService (multipart SMTP)

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

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+package com.condado.newsletter.service
+
+import jakarta.mail.internet.InternetAddress
+import jakarta.mail.internet.MimeMessage
+import org.slf4j.LoggerFactory
+import org.springframework.mail.javamail.JavaMailSender
+import org.springframework.mail.javamail.MimeMessageHelper
+import org.springframework.stereotype.Service
+
+/**
+ * Sends AI-generated emails via SMTP using Spring's [JavaMailSender].
+ * Each email is sent as multipart (text/plain + text/html).
+ */
+@Service
+class EmailSenderService(private val mailSender: JavaMailSender) {
+
+    private val log = LoggerFactory.getLogger(javaClass)
+
+    /**
+     * Sends an email from a virtual entity to all configured recipients.
+     *
+     * @param from Sender email address (the virtual entity's email).
+     * @param to   List of recipient email addresses.
+     * @param subject Email subject line.
+     * @param body Email body (may be plain text or simple HTML).
+     */
+    fun send(from: String, to: List<String>, subject: String, body: String) {
+        log.info("Sending email from='$from' to=$to subject='$subject'")
+        val message: MimeMessage = mailSender.createMimeMessage()
+        val helper = MimeMessageHelper(message, true, "UTF-8")
+        helper.setFrom(InternetAddress(from))
+        helper.setTo(to.toTypedArray())
+        helper.setSubject(subject)
+        // Send as both plain text and HTML for maximum compatibility
+        val plainText = body.replace(Regex("<[^>]+>"), "").trim()
+        val htmlBody = if (body.contains(Regex("<[a-zA-Z]"))) body else "<pre>$body</pre>"
+        helper.setText(plainText, htmlBody)
+        mailSender.send(message)
+        log.info("Email sent successfully from='$from' subject='$subject'")
+    }
+}