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/inttest/email_test.go

@@ -19,8 +19,8 @@ import (
 // recover the confirm-code from the body.
 type capturingMailer struct{ lastBody string }
 
-func (m *capturingMailer) Send(_ context.Context, _, _, body string) error {
-	m.lastBody = body
+func (m *capturingMailer) Send(_ context.Context, msg account.Message) error {
+	m.lastBody = msg.Text
 	return nil
 }
 
@@ -32,7 +32,7 @@ func TestEmailConfirmFlow(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	mailer := &capturingMailer{}
-	svc := account.NewEmailService(store, mailer)
+	svc := account.NewEmailService(store, mailer, "https://erudit-game.ru")
 
 	acc := provisionAccount(t)
 	email := "user-" + uuid.NewString() + "@example.com"
@@ -67,7 +67,7 @@ func TestEmailAlreadyTakenByAnotherAccount(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	mailer := &capturingMailer{}
-	svc := account.NewEmailService(store, mailer)
+	svc := account.NewEmailService(store, mailer, "https://erudit-game.ru")
 
 	owner := provisionAccount(t)
 	email := "taken-" + uuid.NewString() + "@example.com"
@@ -89,7 +89,7 @@ func TestEmailCodeExpires(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	mailer := &capturingMailer{}
-	svc := account.NewEmailService(store, mailer)
+	svc := account.NewEmailService(store, mailer, "https://erudit-game.ru")
 
 	acc := provisionAccount(t)
 	email := "expire-" + uuid.NewString() + "@example.com"
@@ -111,7 +111,7 @@ func TestEmailTooManyAttempts(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	mailer := &capturingMailer{}
-	svc := account.NewEmailService(store, mailer)
+	svc := account.NewEmailService(store, mailer, "https://erudit-game.ru")
 
 	acc := provisionAccount(t)
 	email := "lock-" + uuid.NewString() + "@example.com"
@@ -203,10 +203,10 @@ func TestUpdateProfileOffsetTimezone(t *testing.T) {
 func TestEmailLoginFlow(t *testing.T) {
 	ctx := context.Background()
 	mailer := &capturingMailer{}
-	svc := account.NewEmailService(account.NewStore(testDB), mailer)
+	svc := account.NewEmailService(account.NewStore(testDB), mailer, "https://erudit-game.ru")
 	email := "login-" + uuid.NewString() + "@example.com"
 
-	accountID, err := svc.RequestLoginCode(ctx, email, "+02:00")
+	accountID, err := svc.RequestLoginCode(ctx, email, "+02:00", "en")
 	if err != nil {
 		t.Fatalf("request login code: %v", err)
 	}
@@ -233,7 +233,7 @@ func TestEmailLoginFlow(t *testing.T) {
 	}
 
 	// A second login for the same email is the returning user: same account.
-	if _, err := svc.RequestLoginCode(ctx, email, ""); err != nil {
+	if _, err := svc.RequestLoginCode(ctx, email, "", "ru"); err != nil {
 		t.Fatalf("second request: %v", err)
 	}
 	acc2, err := svc.LoginWithCode(ctx, email, sixDigit.FindString(mailer.lastBody))