feat: tipos y funciones email SMTP en Go (infra)

Tipos: EmailAttachment, EmailMessage, SMTPConfig.
Puras: email_build_html, email_build_text, email_with_attachment, email_template_render.
Impuras: smtp_connect (TLS/STARTTLS/plain), smtp_send (MIME multipart con adjuntos).
Solo stdlib: net/smtp, crypto/tls, text/template, mime/multipart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/email_build_html.go

@@ -0,0 +1,17 @@
+package infra
+
+// EmailBuildHTML constructs an EmailMessage with an HTML body.
+// to is a list of recipient addresses.
+// subject is the email subject line.
+// bodyHTML is the HTML content of the email.
+// Returns a new EmailMessage with no CC, BCC, attachments or custom headers.
+func EmailBuildHTML(from string, to []string, subject, bodyHTML string) EmailMessage {
+	toSlice := make([]string, len(to))
+	copy(toSlice, to)
+	return EmailMessage{
+		From:    from,
+		To:      toSlice,
+		Subject: subject,
+		BodyHTML: bodyHTML,
+	}
+}