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_with_attachment.go

@@ -0,0 +1,38 @@
+package infra
+
+// EmailWithAttachment returns a copy of msg with the given attachment appended.
+// Does not mutate the original message — always returns a new EmailMessage.
+func EmailWithAttachment(msg EmailMessage, att EmailAttachment) EmailMessage {
+	newAtts := make([]EmailAttachment, len(msg.Attachments)+1)
+	copy(newAtts, msg.Attachments)
+	newAtts[len(msg.Attachments)] = att
+
+	toSlice := make([]string, len(msg.To))
+	copy(toSlice, msg.To)
+
+	ccSlice := make([]string, len(msg.CC))
+	copy(ccSlice, msg.CC)
+
+	bccSlice := make([]string, len(msg.BCC))
+	copy(bccSlice, msg.BCC)
+
+	var headers map[string]string
+	if len(msg.Headers) > 0 {
+		headers = make(map[string]string, len(msg.Headers))
+		for k, v := range msg.Headers {
+			headers[k] = v
+		}
+	}
+
+	return EmailMessage{
+		From:        msg.From,
+		To:          toSlice,
+		CC:          ccSlice,
+		BCC:         bccSlice,
+		Subject:     msg.Subject,
+		BodyHTML:    msg.BodyHTML,
+		BodyText:    msg.BodyText,
+		Attachments: newAtts,
+		Headers:     headers,
+	}
+}