email: bound the MIME walk and drop two copies of every body (V-581)

The MIME tree walk had no depth limit, and the nesting comes off the wire.
A boundary line is a few bytes, so one message inside MaxMessageBytes can
declare tens of thousands of multipart levels and pick the recursion depth
of a daemon reading his mail. MaxMIMEDepth stops the walk at 12, well past
the three levels real mail uses, and the headers still come through.

ParseMessage converted the raw message to a string to read it, which copied
up to 2 MiB per mail on a box already holding the resident model. It reads
the bytes directly now. decodeCP1251 collected runes and then copied them
into a string, four bytes a character for the whole body, and writes into a
Builder instead.

No behaviour change to what is read: EXAMINE and BODY.PEEK are still the
only mailbox commands, and no credential reaches a log line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/email/message_test.go

@@ -1,6 +1,7 @@
 package email
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -143,6 +144,24 @@ func TestParseTruncatesLongBody(t *testing.T) {
 	}
 }
 
+// Nesting depth comes off the wire, so a hostile message must not get to pick
+// the recursion depth. The walk stops and the headers still come through.
+func TestParseMessageBoundsMIMEDepth(t *testing.T) {
+	var b strings.Builder
+	b.WriteString("Subject: deep\r\nMIME-Version: 1.0\r\n")
+	for i := 0; i < MaxMIMEDepth+20; i++ {
+		fmt.Fprintf(&b, "Content-Type: multipart/mixed; boundary=\"b%d\"\r\n\r\n--b%d\r\n", i, i)
+	}
+	b.WriteString("Content-Type: text/plain\r\n\r\nглубоко\r\n")
+	msg, err := ParseMessage(7, []byte(b.String()))
+	if err != nil {
+		t.Fatalf("ParseMessage: %v", err)
+	}
+	if msg.Subject != "deep" {
+		t.Errorf("Subject = %q, want the headers to survive", msg.Subject)
+	}
+}
+
 func TestCollapseSqueezesBlankLines(t *testing.T) {
 	got := collapse("  a  b \r\n\r\n\r\n\r\n  c  \r\n")
 	if got != "a b\n\nc" {