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/api/v1/hosts.py

@@ -3,9 +3,16 @@ from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
 from app.auth.jwt_auth import get_current_user
+from app.config import get_settings
 from app.database import get_db
 from app.models import Event, Host
 from app.schemas.list_models import HostListResponse, HostSummary
+from app.services.host_health import (
+    DAILY_REPORT_SSH,
+    HEARTBEAT_TYPE,
+    agent_status,
+    max_event_time_by_host,
+)
 
 router = APIRouter(prefix="/hosts", tags=["hosts"])
 
@@ -37,11 +44,16 @@ def list_hosts(
         .limit(page_size)
     ).all()
 
+    settings = get_settings()
+    hb_map = max_event_time_by_host(db, HEARTBEAT_TYPE)
+    report_map = max_event_time_by_host(db, DAILY_REPORT_SSH)
+
     items: list[HostSummary] = []
     for host in rows:
         event_count = db.scalar(
             select(func.count()).select_from(Event).where(Event.host_id == host.id)
         )
+        last_hb = hb_map.get(host.id)
         items.append(
             HostSummary(
                 id=host.id,
@@ -53,6 +65,11 @@ def list_hosts(
                 ipv4=host.ipv4,
                 last_seen_at=host.last_seen_at,
                 event_count=int(event_count or 0),
+                last_heartbeat_at=last_hb,
+                last_daily_report_at=report_map.get(host.id),
+                agent_status=agent_status(
+                    last_hb, stale_minutes=settings.sac_heartbeat_stale_minutes
+                ),
             )
         )