feat(ticket): record urges in thread, show urge count in admin, mail admins

- aksrt_ticket gains urge_count / last_urge_time (schema 3.1.0)
- cooldown moves from transient to an atomic conditional UPDATE
- urge inserts a system reply rendered as a centered notice (not quotable)
- admin list badge + info card row for urge count / last time
- new ticket_urge_admin mail event via aksrt_ticket_urged

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/css/routes/tickets.css

@@ -545,6 +545,19 @@
 .aksrt-ticket-msg { display: flex; gap: 10px; align-items: flex-start; }
 .aksrt-ticket-msg.is-user { flex-direction: row-reverse; }
 
+/* 系统记录（如催单）：居中灰色提示条。 */
+.aksrt-ticket-msg.is-system { justify-content: center; }
+.aksrt-ticket-msg__system {
+  display: inline-flex;
+  gap: 6px;
+  align-items: center;
+  padding: 3px 12px;
+  border-radius: 999px;
+  background: color-mix(in srgb, var(--tickets-ink) 6%, transparent);
+  color: var(--tickets-muted);
+  font-size: 12px;
+}
+
 .aksrt-ticket-msg__avatar {
   display: inline-grid;
   flex: 0 0 32px;