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/account/mailer_test.go

@@ -1,6 +1,28 @@
 package account
 
-import "testing"
+import (
+	"slices"
+	"testing"
+)
+
+// TestSplitAddrs covers the comma-separated recipient parsing used for the admin alert
+// To (several operator mailboxes in one message), including trimming and empty entries.
+func TestSplitAddrs(t *testing.T) {
+	cases := []struct {
+		in   string
+		want []string
+	}{
+		{"a@x.ru", []string{"a@x.ru"}},
+		{"a@x.ru, b@y.ru", []string{"a@x.ru", "b@y.ru"}},
+		{" a@x.ru ,, b@y.ru ,", []string{"a@x.ru", "b@y.ru"}},
+		{"", nil},
+	}
+	for _, c := range cases {
+		if got := splitAddrs(c.in); !slices.Equal(got, c.want) {
+			t.Errorf("splitAddrs(%q) = %v, want %v", c.in, got, c.want)
+		}
+	}
+}
 
 // TestSMTPTLSMode covers the explicit TLS mode and the port-based fallback, including
 // the non-standard Selectel ports (1127 = SSL, 1126 = STARTTLS) that the 465 heuristic