platform/email: add attachment support; order delegates SMTP to it

go-cart-actor's hand-rolled SMTP stack (SASL loginAuth/plainAuth, dial/
STARTTLS/auth/MAIL/RCPT/DATA wire code, MIME builders) is replaced by an
adapter over platform/email, which gains the multipart/mixed attachment
envelope ported from it. One SMTP implementation now serves both the
order/ledger senders and go-auth's AUTH_EMAIL_* flows; public surfaces
(SMTPEmailSenderConfig, NewSMTPEmailSender, EmailSender, EmailMessage)
are unchanged so all callers compile untouched. Bumps the go-cart-actor
pin to 7cad997.

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Co-Authored-By: Codebuff <noreply@codebuff.com>

email/email.go

@@ -1,10 +1,10 @@
 // Package email is the platform's shared transactional-mail transport: a
 // small Sender interface with an SMTP implementation (implicit TLS, STARTTLS
 // and plain relays; PLAIN/LOGIN auth) and a dev-only log sender. It was
-// extracted from go-cart-actor's pkg/order (where the order, ledger and
-// newsletter paths use it) so a second consumer — go-auth's forgot-password
-// and invite mail — reuses the same transport contract instead of
-// duplicating it.
+// extracted from go-cart-actor's pkg/order — whose SMTPEmailSender now
+// delegates here (order, ledger and newsletter mail flow through it) — and
+// go-auth's forgot-password / invite mail is the second consumer, so every
+// service shares one SMTP implementation.
 //
 // Pure stdlib (net/smtp, crypto/tls, mime, net/mail): no external SDK, so it
 // stays usable from the stdlib-only services. The MailerSend API sender is
@@ -15,6 +15,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/tls"
+	"encoding/base64"
 	"fmt"
 	"log/slog"
 	"mime"
@@ -39,6 +40,16 @@ type Message struct {
 	// Reserved headers (From/To/Subject/Date/MIME-*) are skipped, and CR/LF
 	// injection is rejected.
 	Headers map[string]string
+	// Attachments holds binary payloads (e.g. an invoice PDF). The SMTP
+	// sender wraps the body in a multipart/mixed envelope with one base64
+	// part per attachment.
+	Attachments []Attachment
+}
+
+// Attachment is one binary payload attached to a message.
+type Attachment struct {
+	Filename string
+	Content  []byte
 }
 
 // Sender delivers an already-rendered mail and exposes its default sender
@@ -330,6 +341,22 @@ func buildMessage(msg Message) ([]byte, error) {
 	}
 	writeHeader("MIME-Version", "1.0")
 
+	if len(msg.Attachments) > 0 {
+		// Top level becomes multipart/mixed wrapping the alternative body and
+		// each attachment part.
+		mixed := fmt.Sprintf("mixed-%d", time.Now().UnixNano())
+		writeHeader("Content-Type", fmt.Sprintf(`multipart/mixed; boundary="%s"`, mixed))
+		buf.WriteString("\r\n")
+		writeBodyPart(&buf, mixed, msg)
+		for _, a := range msg.Attachments {
+			writeAttachmentPart(&buf, mixed, a)
+		}
+		buf.WriteString("--")
+		buf.WriteString(mixed)
+		buf.WriteString("--\r\n")
+		return buf.Bytes(), nil
+	}
+
 	switch {
 	case msg.TextBody != "" && msg.HTMLBody != "":
 		boundary := fmt.Sprintf("alt-%d", time.Now().UnixNano())
@@ -358,6 +385,45 @@ func buildMessage(msg Message) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
+// writeBodyPart writes the text/html alternative body as one part inside a
+// multipart/mixed envelope.
+func writeBodyPart(buf *bytes.Buffer, boundary string, msg Message) {
+	alt := fmt.Sprintf("alt-%d", time.Now().UnixNano())
+	buf.WriteString("--")
+	buf.WriteString(boundary)
+	buf.WriteString("\r\n")
+	buf.WriteString("Content-Type: ")
+	buf.WriteString(fmt.Sprintf(`multipart/alternative; boundary="%s"`, alt))
+	buf.WriteString("\r\n\r\n")
+	if msg.TextBody != "" {
+		writeMIMEPart(buf, alt, "text/plain; charset=utf-8", msg.TextBody)
+	}
+	if msg.HTMLBody != "" {
+		writeMIMEPart(buf, alt, "text/html; charset=utf-8", msg.HTMLBody)
+	}
+	buf.WriteString("--")
+	buf.WriteString(alt)
+	buf.WriteString("--\r\n")
+}
+
+// writeAttachmentPart writes one base64 attachment part.
+func writeAttachmentPart(buf *bytes.Buffer, boundary string, a Attachment) {
+	buf.WriteString("--")
+	buf.WriteString(boundary)
+	buf.WriteString("\r\n")
+	buf.WriteString("Content-Type: application/pdf; name=\"")
+	buf.WriteString(a.Filename)
+	buf.WriteString("\"\r\n")
+	buf.WriteString("Content-Transfer-Encoding: base64\r\n")
+	buf.WriteString("Content-Disposition: attachment; filename=\"")
+	buf.WriteString(a.Filename)
+	buf.WriteString("\"\r\n\r\n")
+	w := base64.NewEncoder(base64.StdEncoding, buf)
+	_, _ = w.Write(a.Content)
+	_ = w.Close()
+	buf.WriteString("\r\n")
+}
+
 func writeMIMEPart(buf *bytes.Buffer, boundary, contentType, body string) {
 	buf.WriteString("--")
 	buf.WriteString(boundary)