feat(account): brand and harden the email pipeline

Swap the net/smtp mailer for go-mail behind the existing Mailer seam: the
Message struct now carries a text + HTML body, TLS mode is chosen from the port
(implicit TLS on 465, else mandatory STARTTLS), a dial timeout bounds the
synchronous send, and no client certificate is needed. Add a branded, image-free,
mobile-friendly ru/en HTML template (with a plain-text alternative) rendering a
large readable code and an ignore-notice footer with a landing link.

Add BACKEND_PUBLIC_BASE_URL config (the canonical origin for the email footer
link, never the request Host — anti-injection), required when a relay is
configured.

Fix the email-login address squat: ProvisionEmail creates the account flagged
is_guest until the code is confirmed, so an abandoned login is reaped like any
guest and its address freed; confirming (login or link) clears the flag. Seed the
new account's language from the client, plumbed through the email-login request.

The confirm deeplink, its transport surface and the send rate-limit land in
follow-up work.

backend/internal/account/mailer.go

@@ -3,23 +3,37 @@ package account
 import (
 	"context"
 	"fmt"
-	"net"
-	"net/smtp"
+	"strconv"
+	"time"
 
+	"github.com/wneessen/go-mail"
 	"go.uber.org/zap"
 )
 
+// Message is a transactional email to send through a Mailer. Text is the
+// required plain-text body and doubles as the multipart/alternative fallback;
+// HTML, when non-empty, is the preferred body a capable client renders instead.
+type Message struct {
+	To      string
+	Subject string
+	Text    string
+	HTML    string
+}
+
 // Mailer delivers a transactional email. It is the seam behind which the email
 // confirm-code flow sends codes, so the relay is swappable and unit tests use a
-// fixture (see docs/TESTING.md: no real network in tests). The context is offered
-// for cancellation; the standard-library SMTP implementation sends synchronously
-// and ignores it.
+// fixture (see docs/TESTING.md: no real network in tests). The context bounds the
+// delivery and is honoured by the SMTP implementation.
 type Mailer interface {
-	Send(ctx context.Context, to, subject, body string) error
+	Send(ctx context.Context, msg Message) error
 }
 
 // SMTPConfig configures the SMTP relay. An empty Host selects the LogMailer
 // instead, so a deployment without a relay still runs (the code lands in the log).
+// TLS is always used and no client certificate is required — only the server
+// certificate is validated against the system roots. The Port selects the TLS
+// mode: smtpImplicitTLSPort (465) uses implicit TLS (SSL); any other port uses
+// mandatory STARTTLS.
 type SMTPConfig struct {
 	Host     string
 	Port     string
@@ -28,8 +42,19 @@ type SMTPConfig struct {
 	From     string
 }
 
-// SMTPMailer sends mail through an SMTP relay using the standard library. When a
-// username is set it authenticates with PLAIN; otherwise it relays unauthenticated.
+const (
+	// smtpImplicitTLSPort is the port on which the relay is dialled with implicit
+	// TLS (SSL). Any other port negotiates mandatory STARTTLS instead.
+	smtpImplicitTLSPort = 465
+	// smtpDialTimeout bounds a single relay connect-and-send. The confirm-code send
+	// is synchronous on the request path, so an unreachable relay must fail fast
+	// rather than hold the request open.
+	smtpDialTimeout = 15 * time.Second
+)
+
+// SMTPMailer sends mail through an SMTP relay using go-mail. When a username is
+// set it authenticates, auto-discovering the strongest mechanism the relay
+// advertises; otherwise it relays unauthenticated.
 type SMTPMailer struct {
 	cfg SMTPConfig
 }
@@ -39,29 +64,49 @@ func NewSMTPMailer(cfg SMTPConfig) SMTPMailer {
 	return SMTPMailer{cfg: cfg}
 }
 
-// Send delivers a plain-text UTF-8 message to to via the configured relay.
-func (m SMTPMailer) Send(_ context.Context, to, subject, body string) error {
-	addr := net.JoinHostPort(m.cfg.Host, m.cfg.Port)
-	var auth smtp.Auth
-	if m.cfg.Username != "" {
-		auth = smtp.PlainAuth("", m.cfg.Username, m.cfg.Password, m.cfg.Host)
+// Send delivers a UTF-8 message to msg.To via the configured relay. When msg.HTML
+// is set the message is multipart/alternative (plain text plus HTML); otherwise
+// it is plain text only.
+func (m SMTPMailer) Send(ctx context.Context, msg Message) error {
+	port, err := strconv.Atoi(m.cfg.Port)
+	if err != nil {
+		return fmt.Errorf("account: invalid SMTP port %q: %w", m.cfg.Port, err)
 	}
-	if err := smtp.SendMail(addr, auth, m.cfg.From, []string{to}, message(m.cfg.From, to, subject, body)); err != nil {
-		return fmt.Errorf("account: send mail to %s: %w", to, err)
+	opts := []mail.Option{mail.WithTimeout(smtpDialTimeout)}
+	if port == smtpImplicitTLSPort {
+		opts = append(opts, mail.WithSSLPort(false))
+	} else {
+		opts = append(opts, mail.WithPort(port), mail.WithTLSPortPolicy(mail.TLSMandatory))
+	}
+	if m.cfg.Username != "" {
+		opts = append(opts,
+			mail.WithSMTPAuth(mail.SMTPAuthAutoDiscover),
+			mail.WithUsername(m.cfg.Username),
+			mail.WithPassword(m.cfg.Password),
+		)
+	}
+	client, err := mail.NewClient(m.cfg.Host, opts...)
+	if err != nil {
+		return fmt.Errorf("account: build mail client: %w", err)
+	}
+	out := mail.NewMsg()
+	if err := out.From(m.cfg.From); err != nil {
+		return fmt.Errorf("account: set From %q: %w", m.cfg.From, err)
+	}
+	if err := out.To(msg.To); err != nil {
+		return fmt.Errorf("account: set To %q: %w", msg.To, err)
+	}
+	out.Subject(msg.Subject)
+	out.SetBodyString(mail.TypeTextPlain, msg.Text)
+	if msg.HTML != "" {
+		out.AddAlternativeString(mail.TypeTextHTML, msg.HTML)
+	}
+	if err := client.DialAndSendWithContext(ctx, out); err != nil {
+		return fmt.Errorf("account: send mail to %s: %w", msg.To, err)
 	}
 	return nil
 }
 
-// message renders a minimal RFC 5322 plain-text email.
-func message(from, to, subject, body string) []byte {
-	return []byte("From: " + from + "\r\n" +
-		"To: " + to + "\r\n" +
-		"Subject: " + subject + "\r\n" +
-		"MIME-Version: 1.0\r\n" +
-		"Content-Type: text/plain; charset=UTF-8\r\n" +
-		"\r\n" + body + "\r\n")
-}
-
 // LogMailer logs the message instead of sending it. It is the default when no
 // SMTP relay is configured and is intended for development only: it logs the body,
 // which carries the confirm-code, so it must not be used in production.
@@ -74,11 +119,12 @@ func NewLogMailer(log *zap.Logger) LogMailer {
 	return LogMailer{log: log}
 }
 
-// Send logs the message at info level and reports success.
-func (m LogMailer) Send(_ context.Context, to, subject, body string) error {
+// Send logs the message at info level and reports success. It logs the plain-text
+// body only (which carries the confirm-code); the HTML alternative is omitted.
+func (m LogMailer) Send(_ context.Context, msg Message) error {
 	if m.log != nil {
 		m.log.Info("email not sent (log mailer)",
-			zap.String("to", to), zap.String("subject", subject), zap.String("body", body))
+			zap.String("to", msg.To), zap.String("subject", msg.Subject), zap.String("body", msg.Text))
 	}
 	return nil
 }