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/schemas/list_models.py

@@ -13,6 +13,9 @@ class HostSummary(BaseModel):
     ipv4: str | None
     last_seen_at: datetime
     event_count: int | None = None
+    last_heartbeat_at: datetime | None = None
+    last_daily_report_at: datetime | None = None
+    agent_status: str = "unknown"
 
     model_config = {"from_attributes": True}