feat: host heartbeat status and dashboard metrics (2.2)

Track agent.heartbeat and report.daily.ssh per host; show online/stale
on Hosts UI; dashboard counters and SAC_HEARTBEAT_STALE_MINUTES config.

backend/app/config.py

@@ -50,6 +50,9 @@ class Settings(BaseSettings):
     telegram_chat_id: str = ""
     telegram_min_severity: str = "high"
 
+    # Порог «живости» агента: agent.heartbeat раз в ~12 ч (ssh-monitor)
+    sac_heartbeat_stale_minutes: int = 780
+
 
 @lru_cache
 def get_settings() -> Settings: