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/smtp_send.go

@@ -0,0 +1,154 @@
+package infra
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"mime"
+	"mime/multipart"
+	"net/smtp"
+	"net/textproto"
+	"strings"
+	"time"
+)
+
+// SMTPSend serializes msg to MIME and sends it using the given smtp.Client.
+// Supports plain text, HTML, and mixed (with attachments) content types.
+// The client must be authenticated (obtained from SMTPConnect).
+func SMTPSend(client *smtp.Client, msg EmailMessage) error {
+	if err := client.Mail(extractAddr(msg.From)); err != nil {
+		return fmt.Errorf("smtp_send: MAIL FROM: %w", err)
+	}
+
+	allTo := append(append([]string{}, msg.To...), msg.CC...)
+	allTo = append(allTo, msg.BCC...)
+	for _, addr := range allTo {
+		if err := client.Rcpt(extractAddr(addr)); err != nil {
+			return fmt.Errorf("smtp_send: RCPT TO %s: %w", addr, err)
+		}
+	}
+
+	w, err := client.Data()
+	if err != nil {
+		return fmt.Errorf("smtp_send: DATA: %w", err)
+	}
+	defer w.Close()
+
+	body, err := buildMIME(msg)
+	if err != nil {
+		return fmt.Errorf("smtp_send: build mime: %w", err)
+	}
+
+	if _, err := w.Write(body); err != nil {
+		return fmt.Errorf("smtp_send: write: %w", err)
+	}
+	return nil
+}
+
+// extractAddr extracts the bare email address from a possibly formatted string
+// like "Alice <alice@example.com>" → "alice@example.com".
+func extractAddr(addr string) string {
+	if i := strings.Index(addr, "<"); i >= 0 {
+		if j := strings.Index(addr[i:], ">"); j >= 0 {
+			return addr[i+1 : i+j]
+		}
+	}
+	return strings.TrimSpace(addr)
+}
+
+// buildMIME constructs the full MIME email bytes.
+func buildMIME(msg EmailMessage) ([]byte, error) {
+	var buf bytes.Buffer
+
+	// Headers
+	buf.WriteString("Date: " + time.Now().UTC().Format(time.RFC1123Z) + "\r\n")
+	buf.WriteString("From: " + msg.From + "\r\n")
+	buf.WriteString("To: " + strings.Join(msg.To, ", ") + "\r\n")
+	if len(msg.CC) > 0 {
+		buf.WriteString("Cc: " + strings.Join(msg.CC, ", ") + "\r\n")
+	}
+	buf.WriteString("Subject: " + mime.QEncoding.Encode("utf-8", msg.Subject) + "\r\n")
+	buf.WriteString("MIME-Version: 1.0\r\n")
+	for k, v := range msg.Headers {
+		buf.WriteString(k + ": " + v + "\r\n")
+	}
+
+	hasText := msg.BodyText != ""
+	hasHTML := msg.BodyHTML != ""
+	hasAtts := len(msg.Attachments) > 0
+
+	switch {
+	case hasAtts:
+		// multipart/mixed: text or html parts + attachments
+		mw := multipart.NewWriter(&buf)
+		buf.WriteString("Content-Type: multipart/mixed; boundary=\"" + mw.Boundary() + "\"\r\n\r\n")
+
+		if hasText && hasHTML {
+			// nested multipart/alternative inside mixed
+			var altBuf bytes.Buffer
+			aw := multipart.NewWriter(&altBuf)
+			writeTextPart(aw, msg.BodyText)
+			writeHTMLPart(aw, msg.BodyHTML)
+			aw.Close()
+			altHeader := textproto.MIMEHeader{}
+			altHeader.Set("Content-Type", "multipart/alternative; boundary=\""+aw.Boundary()+"\"")
+			pw, _ := mw.CreatePart(altHeader)
+			pw.Write(altBuf.Bytes())
+		} else if hasHTML {
+			writeHTMLPart(mw, msg.BodyHTML)
+		} else {
+			writeTextPart(mw, msg.BodyText)
+		}
+
+		for _, att := range msg.Attachments {
+			writeAttachment(mw, att)
+		}
+		mw.Close()
+
+	case hasText && hasHTML:
+		mw := multipart.NewWriter(&buf)
+		buf.WriteString("Content-Type: multipart/alternative; boundary=\"" + mw.Boundary() + "\"\r\n\r\n")
+		writeTextPart(mw, msg.BodyText)
+		writeHTMLPart(mw, msg.BodyHTML)
+		mw.Close()
+
+	case hasHTML:
+		buf.WriteString("Content-Type: text/html; charset=utf-8\r\n")
+		buf.WriteString("Content-Transfer-Encoding: quoted-printable\r\n\r\n")
+		buf.WriteString(msg.BodyHTML)
+
+	default:
+		buf.WriteString("Content-Type: text/plain; charset=utf-8\r\n")
+		buf.WriteString("Content-Transfer-Encoding: quoted-printable\r\n\r\n")
+		buf.WriteString(msg.BodyText)
+	}
+
+	return buf.Bytes(), nil
+}
+
+func writeTextPart(mw *multipart.Writer, body string) {
+	h := textproto.MIMEHeader{}
+	h.Set("Content-Type", "text/plain; charset=utf-8")
+	h.Set("Content-Transfer-Encoding", "quoted-printable")
+	pw, _ := mw.CreatePart(h)
+	pw.Write([]byte(body))
+}
+
+func writeHTMLPart(mw *multipart.Writer, body string) {
+	h := textproto.MIMEHeader{}
+	h.Set("Content-Type", "text/html; charset=utf-8")
+	h.Set("Content-Transfer-Encoding", "quoted-printable")
+	pw, _ := mw.CreatePart(h)
+	pw.Write([]byte(body))
+}
+
+func writeAttachment(mw *multipart.Writer, att EmailAttachment) {
+	h := textproto.MIMEHeader{}
+	h.Set("Content-Type", att.ContentType)
+	h.Set("Content-Transfer-Encoding", "base64")
+	h.Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"`, att.Filename))
+	pw, _ := mw.CreatePart(h)
+	enc := base64.NewEncoder(base64.StdEncoding, pw)
+	enc.Write(att.Data)
+	enc.Close()
+}