feat: proactive host_silence scan for stale agent.heartbeat (0.8.3)

Background scan every 5 min opens rule:host_silence and notifies without
waiting for the next ingest event. CLI job + optional systemd timer.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/telegram_templates.py

@@ -543,6 +543,8 @@ def format_problem_telegram_html(problem: Problem, event: Event | None = None) -
     msg += f"\n<b>{html_escape(problem.title)}</b>\n{html_escape(problem.summary)}"
     if event is not None:
         msg += f"\n\n<i>Триггер:</i> {html_escape(event.type)} ({html_escape(event.severity)})"
+    elif problem.rule_id == "rule:host_silence":
+        msg += "\n\n<i>Триггер:</i> периодическая проверка SAC (host_silence scan)"
         if event.type in (
             "rdp.login.success",
             "rdp.login.failed",