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/stream.py

@@ -10,6 +10,8 @@ from sqlalchemy.orm import Session
 from app.auth.jwt_auth import verify_access_token
 from app.database import SessionLocal
 from app.models import Event, Problem
+from app.config import get_settings
+from app.services.host_health import DAILY_REPORT_SSH, HEARTBEAT_TYPE, count_stale_hosts
 from app.version import APP_VERSION
 
 router = APIRouter(prefix="/stream", tags=["stream"])
@@ -26,10 +28,24 @@ def _dashboard_snapshot(db: Session) -> dict:
         db.scalar(select(func.count()).select_from(Problem).where(Problem.status == "open")) or 0
     )
     last_event = db.scalar(select(Event.id).order_by(Event.received_at.desc()).limit(1))
+    settings = get_settings()
     return {
         "type": "dashboard",
         "version": APP_VERSION,
         "events_last_24h": events_24h,
+        "hosts_stale": count_stale_hosts(db, settings.sac_heartbeat_stale_minutes),
+        "heartbeats_24h": db.scalar(
+            select(func.count())
+            .select_from(Event)
+            .where(Event.received_at >= since, Event.type == HEARTBEAT_TYPE)
+        )
+        or 0,
+        "daily_reports_24h": db.scalar(
+            select(func.count())
+            .select_from(Event)
+            .where(Event.received_at >= since, Event.type == DAILY_REPORT_SSH)
+        )
+        or 0,
         "problems_open": problems_open,
         "last_event_id": last_event,
         "at": datetime.now(timezone.utc).isoformat(),