feat(email): PWA login sends code only; drop admin link from alert emails

Two email changes, per the owner:

1. Code-only login from an installed PWA. A login requested while running as a
   standalone PWA now omits the one-tap confirm link from the email — the link
   would open in a separate browser whose minted session cannot reach the PWA,
   stranding the login. The code is typed in the same window instead. The client
   sends a `pwa` flag (isStandalone) on the email-code request (a new FBS field,
   threaded through the gateway); the backend omits the deeplink when it is set,
   reusing the existing deletion-code link-omission path. Non-PWA browser logins
   keep the one-tap link. No polling, no migration.

2. Security: the operator alert digest no longer embeds the admin-console (/_gm)
   URL. An admin link must never travel in an email, where a mail provider could
   cache or index it; the operator opens the console directly.

Tests: an inttest asserting the PWA login email omits the link (and a browser one
keeps it); a codec round-trip of the new pwa field; the alert-digest test flipped
to guard the admin link is absent. Docs: ARCHITECTURE + FUNCTIONAL (+ru).

backend/internal/adminalert/notify.go

@@ -33,21 +33,21 @@ type Notifier struct {
 	complaints ComplaintCounter
 	from       string
 	to         string
-	consoleURL string
 	clock      func() time.Time
 	log        *zap.Logger
 	last       time.Time
 }
 
-// New constructs a Notifier. from and to are the alert sender and recipient(s); consoleURL,
-// when non-empty, is the admin-console link included in the email. log may be nil. The
-// watermark starts at "now", so only items arriving after start-up are reported.
-func New(mailer account.Mailer, fb FeedbackCounter, cp ComplaintCounter, from, to, consoleURL string, log *zap.Logger) *Notifier {
+// New constructs a Notifier. from and to are the alert sender and recipient(s); log may be
+// nil. The watermark starts at "now", so only items arriving after start-up are reported. The
+// digest deliberately carries no admin-console link — an admin URL must never travel in an
+// email, where a mail provider could cache or index it.
+func New(mailer account.Mailer, fb FeedbackCounter, cp ComplaintCounter, from, to string, log *zap.Logger) *Notifier {
 	if log == nil {
 		log = zap.NewNop()
 	}
 	return &Notifier{
-		mailer: mailer, feedback: fb, complaints: cp, from: from, to: to, consoleURL: consoleURL,
+		mailer: mailer, feedback: fb, complaints: cp, from: from, to: to,
 		clock: func() time.Time { return time.Now().UTC() }, log: log, last: time.Now().UTC(),
 	}
 }
@@ -103,10 +103,9 @@ func (n *Notifier) digest(fb, cp int) account.Message {
 		parts = append(parts, fmt.Sprintf("%d new word complaint(s)", cp))
 	}
 	summary := strings.Join(parts, ", ")
+	// No admin-console link in the body: an admin URL must never travel in an email (a mail
+	// provider could cache or index it). The operator opens the console directly.
 	text := summary + "."
-	if n.consoleURL != "" {
-		text += "\n\nOpen the admin console: " + n.consoleURL
-	}
 	return account.Message{
 		From:    n.from,
 		To:      n.to,

backend/internal/adminalert/notify_test.go

@@ -28,7 +28,7 @@ func (m *recordingMailer) Send(_ context.Context, msg account.Message) error {
 
 func TestNotifierSkipsWhenNothingNew(t *testing.T) {
 	mailer := &recordingMailer{}
-	n := New(mailer, fbCounter{0}, cpCounter{0}, "alerts@erudit-game.ru", "op@x.ru", "", nil)
+	n := New(mailer, fbCounter{0}, cpCounter{0}, "alerts@erudit-game.ru", "op@x.ru", nil)
 	n.tick(context.Background())
 	if len(mailer.sent) != 0 {
 		t.Fatalf("sent %d emails, want 0 when nothing is new", len(mailer.sent))
@@ -37,7 +37,7 @@ func TestNotifierSkipsWhenNothingNew(t *testing.T) {
 
 func TestNotifierDigestsNewItems(t *testing.T) {
 	mailer := &recordingMailer{}
-	n := New(mailer, fbCounter{2}, cpCounter{1}, "alerts@erudit-game.ru", "op@x.ru, two@x.ru", "https://erudit-game.ru/_gm", nil)
+	n := New(mailer, fbCounter{2}, cpCounter{1}, "alerts@erudit-game.ru", "op@x.ru, two@x.ru", nil)
 	n.tick(context.Background())
 	if len(mailer.sent) != 1 {
 		t.Fatalf("sent %d emails, want 1 digest", len(mailer.sent))
@@ -49,7 +49,9 @@ func TestNotifierDigestsNewItems(t *testing.T) {
 	if !strings.Contains(msg.Subject, "2 new feedback") || !strings.Contains(msg.Subject, "1 new word complaint") {
 		t.Errorf("digest subject = %q, want the feedback + complaint counts", msg.Subject)
 	}
-	if !strings.Contains(msg.Text, "/_gm") {
-		t.Errorf("digest body = %q, want the console link", msg.Text)
+	// The digest must never carry an admin-console link — an admin URL in an email is a leak
+	// (mail providers cache/index it).
+	if strings.Contains(msg.Text, "/_gm") || strings.Contains(strings.ToLower(msg.Text), "admin console") {
+		t.Errorf("digest body = %q, must not carry an admin-console link", msg.Text)
 	}
 }