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/config/config.go

@@ -4,6 +4,7 @@ package config
 
 import (
 	"fmt"
+	"net/url"
 	"os"
 	"strconv"
 	"time"
@@ -42,6 +43,12 @@ type Config struct {
 	// SMTP configures the email relay used for confirm-codes. An empty Host
 	// selects the development log mailer (the code is logged, not sent).
 	SMTP account.SMTPConfig
+	// PublicBaseURL is the canonical public origin (scheme + host, e.g.
+	// https://erudit-game.ru) used to build absolute links in outgoing email — the
+	// confirm deeplink and the footer landing link. It is deliberately not derived
+	// from a request Host header, which would let an attacker inject a phishing link
+	// into the email. Required whenever an SMTP relay is configured.
+	PublicBaseURL string
 	// ConnectorAddr is the gRPC address of the Telegram platform connector
 	// side-service, used by the admin console to send operator broadcasts. Empty
 	// disables broadcasts (the admin broadcast actions report "not configured").
@@ -154,6 +161,7 @@ func Load() (Config, error) {
 		Robot:             rb,
 		RateWatch:         rw,
 		SMTP:              smtp,
+		PublicBaseURL:     os.Getenv("BACKEND_PUBLIC_BASE_URL"),
 		ConnectorAddr:     os.Getenv("BACKEND_CONNECTOR_ADDR"),
 		GuestReapInterval: guestReapInterval,
 		GuestRetention:    guestRetention,
@@ -203,6 +211,14 @@ func (c Config) validate() error {
 	if c.GuestRetention <= 0 {
 		return fmt.Errorf("config: BACKEND_GUEST_RETENTION must be positive")
 	}
+	if c.SMTP.Host != "" {
+		if c.PublicBaseURL == "" {
+			return fmt.Errorf("config: BACKEND_PUBLIC_BASE_URL must be set when BACKEND_SMTP_HOST is configured")
+		}
+		if u, err := url.Parse(c.PublicBaseURL); err != nil || u.Scheme == "" || u.Host == "" {
+			return fmt.Errorf("config: BACKEND_PUBLIC_BASE_URL %q must be an absolute URL (scheme://host)", c.PublicBaseURL)
+		}
+	}
 	return nil
 }