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/services/host_health.py

@@ -0,0 +1,53 @@
+"""Статус агентов по событиям agent.heartbeat / report.daily.* в БД."""
+
+from datetime import datetime, timezone
+
+from sqlalchemy import func, select
+from sqlalchemy.orm import Session
+
+from app.models import Event, Host
+
+HEARTBEAT_TYPE = "agent.heartbeat"
+DAILY_REPORT_SSH = "report.daily.ssh"
+
+
+def max_event_time_by_host(db: Session, event_type: str) -> dict[int, datetime]:
+    rows = db.execute(
+        select(Event.host_id, func.max(Event.received_at))
+        .where(Event.type == event_type)
+        .group_by(Event.host_id)
+    ).all()
+    return {int(host_id): ts for host_id, ts in rows if ts is not None}
+
+
+def agent_status(
+    last_heartbeat_at: datetime | None,
+    *,
+    stale_minutes: int,
+    now: datetime | None = None,
+) -> str:
+    """
+    online — heartbeat не старше порога;
+    stale — был хост, но heartbeat устарел или отсутствует;
+    unknown — heartbeat ещё не приходил.
+    """
+    if last_heartbeat_at is None:
+        return "unknown"
+    ref = now or datetime.now(timezone.utc)
+    hb = last_heartbeat_at
+    if hb.tzinfo is None:
+        hb = hb.replace(tzinfo=timezone.utc)
+    age_sec = (ref - hb).total_seconds()
+    if age_sec > stale_minutes * 60:
+        return "stale"
+    return "online"
+
+
+def count_stale_hosts(db: Session, stale_minutes: int) -> int:
+    hb_map = max_event_time_by_host(db, HEARTBEAT_TYPE)
+    host_ids = db.scalars(select(Host.id)).all()
+    return sum(
+        1
+        for host_id in host_ids
+        if agent_status(hb_map.get(host_id), stale_minutes=stale_minutes) == "stale"
+    )