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/dashboards.py

@@ -6,9 +6,11 @@ from sqlalchemy import func, select
 from sqlalchemy.orm import Session, joinedload
 
 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, Problem
 from app.schemas.list_models import EventSummary
+from app.services.host_health import DAILY_REPORT_SSH, HEARTBEAT_TYPE, count_stale_hosts
 
 router = APIRouter(prefix="/dashboards", tags=["dashboards"])
 
@@ -16,6 +18,9 @@ router = APIRouter(prefix="/dashboards", tags=["dashboards"])
 class DashboardSummary(BaseModel):
     events_last_24h: int
     hosts_total: int
+    hosts_stale: int
+    heartbeats_24h: int
+    daily_reports_24h: int
     problems_open: int
     severity_24h: dict[str, int]
     recent_events: list[EventSummary]
@@ -32,6 +37,18 @@ def dashboard_summary(
         select(func.count()).select_from(Event).where(Event.received_at >= since)
     ) or 0
     hosts_total = db.scalar(select(func.count()).select_from(Host)) or 0
+    settings = get_settings()
+    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 = (
         db.scalar(select(func.count()).select_from(Problem).where(Problem.status == "open")) or 0
     )
@@ -71,6 +88,9 @@ def dashboard_summary(
     return DashboardSummary(
         events_last_24h=events_24h,
         hosts_total=hosts_total,
+        hosts_stale=hosts_stale,
+        heartbeats_24h=heartbeats_24h,
+        daily_reports_24h=daily_reports_24h,
         problems_open=problems_open,
         severity_24h=severity_24h,
         recent_events=recent_events,