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/config.py

@@ -85,6 +85,10 @@ class Settings(BaseSettings):
     # Порог «живости» агента
     sac_heartbeat_stale_minutes: int = 780
 
+    # Периодическое сканирование stale heartbeat (proactive host_silence)
+    sac_host_silence_scan_enabled: bool = True
+    sac_host_silence_scan_interval_minutes: int = 5
+
     # Окно корреляции Problems: host + type + rule в одном open Problem
     sac_problem_correlation_window_minutes: int = 60