test(04-04): close mail safety and Mailpit SMTP receipt coverage

- Assert memory recipients, HTML safety, and SMTP TLS without credential leaks
- Prove real SMTP delivery through Mailpit HTTP API when Docker is available
- Add scripts/check-phase4.sh as the phase vet, test, race, and SMTP gate

Co-authored-by: Cursor <cursoragent@cursor.com>

postcard/smtp_test.go

@@ -0,0 +1,65 @@
+package postcard
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSMTPTLSPolicy(t *testing.T) {
+	t.Parallel()
+
+	for _, tls := range []string{"", "mandatory", "tls", "none", "notls", "starttls", "opportunistic"} {
+		_, err := NewSMTPDriver(SMTPConfig{Host: "127.0.0.1", Port: 1025, TLS: tls, Timeout: time.Second})
+		if err != nil {
+			t.Fatalf("tls %q: %v", tls, err)
+		}
+	}
+
+	_, err := NewSMTPDriver(SMTPConfig{Host: "127.0.0.1", TLS: "maybe"})
+	if err == nil || !strings.Contains(err.Error(), "mail.smtp.tls") {
+		t.Fatalf("unknown tls error = %v", err)
+	}
+	_, err = NewSMTPDriver(SMTPConfig{TLS: "none"})
+	if err == nil || !strings.Contains(err.Error(), "mail.smtp.host") {
+		t.Fatalf("empty host error = %v", err)
+	}
+	_, err = NewSMTPDriver(SMTPConfig{Host: "   ", TLS: "mandatory"})
+	if err == nil || !strings.Contains(err.Error(), "mail.smtp.host") {
+		t.Fatalf("blank host error = %v", err)
+	}
+}
+
+func TestSMTPErrorsOmitCredentialsAndBodies(t *testing.T) {
+	cat := smokeCatalog(t)
+	d, err := NewSMTPDriver(SMTPConfig{
+		Host:     "127.0.0.1",
+		Port:     1,
+		Username: "user",
+		Password: "secretpass",
+		TLS:      "none",
+		Timeout:  80 * time.Millisecond,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	mail := NewMailer(cat, d, Options{From: "from@example.test"})
+	err = mail.Send(context.Background(), Message{
+		Template: "golem15.hello::mail.hello",
+		To:       []string{"ada@example.test"},
+		Vars:     map[string]any{"Name": "Ada", "URL": "https://example.test"},
+	})
+	if err == nil {
+		t.Fatal("expected smtp failure")
+	}
+	msg := err.Error()
+	if !strings.Contains(msg, "smtp") {
+		t.Fatalf("error %q should mention smtp", msg)
+	}
+	for _, leaked := range []string{"secretpass", "Witaj **Ada**.", "text body"} {
+		if strings.Contains(msg, leaked) {
+			t.Fatalf("smtp error leaked %q: %v", leaked, err)
+		}
+	}
+}