feat(adminalert): operator email on new feedback / word complaints

New adminalert worker polls for feedback + word complaints arriving since the last check
and coalesces a burst into one digest email per interval (5 min), inert unless a distinct
admin sender (BACKEND_SMTP_ADMIN_FROM) and recipient (BACKEND_ADMIN_EMAIL, comma-separated
allowed) are configured. The mailer gains a per-message From override and splits a
comma-separated To into separate recipients (go-mail needs them as a list). Feedback/game
stores gain CountSince/CountComplaintsSince. Unit tests cover the digest, the skip-when-
empty, and the recipient split.

backend/internal/adminalert/notify_test.go

@@ -0,0 +1,55 @@
+package adminalert
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+
+	"scrabble/backend/internal/account"
+)
+
+// The fakes ignore the watermark and return a fixed count, which is all the digest logic
+// needs.
+type fbCounter struct{ n int }
+
+func (f fbCounter) CountSince(context.Context, time.Time) (int, error) { return f.n, nil }
+
+type cpCounter struct{ n int }
+
+func (c cpCounter) CountComplaintsSince(context.Context, time.Time) (int, error) { return c.n, nil }
+
+type recordingMailer struct{ sent []account.Message }
+
+func (m *recordingMailer) Send(_ context.Context, msg account.Message) error {
+	m.sent = append(m.sent, msg)
+	return nil
+}
+
+func TestNotifierSkipsWhenNothingNew(t *testing.T) {
+	mailer := &recordingMailer{}
+	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))
+	}
+}
+
+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.tick(context.Background())
+	if len(mailer.sent) != 1 {
+		t.Fatalf("sent %d emails, want 1 digest", len(mailer.sent))
+	}
+	msg := mailer.sent[0]
+	if msg.From != "alerts@erudit-game.ru" || msg.To != "op@x.ru, two@x.ru" {
+		t.Errorf("digest addressing = From %q To %q", msg.From, msg.To)
+	}
+	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)
+	}
+}