feat: proactive host_silence scan for stale agent.heartbeat (0.8.3)

Background scan every 5 min opens rule:host_silence and notifies without
waiting for the next ingest event. CLI job + optional systemd timer.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/jobs/host_silence_scan.py

@@ -0,0 +1,38 @@
+"""CLI: python -m app.jobs.host_silence_scan"""
+
+from __future__ import annotations
+
+import logging
+import sys
+
+from app.database import SessionLocal
+from app.services.host_silence_scan import dispatch_host_silence_notifications, run_host_silence_scan
+
+logging.basicConfig(level=logging.INFO, format="%(asctime)s %(levelname)s %(message)s")
+logger = logging.getLogger("sac.host_silence_scan")
+
+
+def main() -> int:
+    db = SessionLocal()
+    try:
+        results = run_host_silence_scan(db)
+        notified = dispatch_host_silence_notifications(db, results)
+        db.commit()
+        created = sum(1 for r in results if r.created)
+        logger.info(
+            "host_silence scan done stale_hosts=%s created=%s notified=%s",
+            len(results),
+            created,
+            notified,
+        )
+        return 0
+    except Exception:
+        db.rollback()
+        logger.exception("host_silence scan failed")
+        return 1
+    finally:
+        db.close()
+
+
+if __name__ == "__main__":
+    sys.exit(main())