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/main.py

@@ -1,5 +1,6 @@
+import asyncio
 import logging
-from contextlib import asynccontextmanager
+from contextlib import asynccontextmanager, suppress
 from pathlib import Path
 
 from fastapi import FastAPI, HTTPException
@@ -57,7 +58,22 @@ async def lifespan(_app: FastAPI):
     logger.info("%s — application startup (version %s)", APP_VERSION_LABEL, APP_VERSION)
     bootstrap_api_key()
     bootstrap_users()
+
+    stop_scan = asyncio.Event()
+    scan_task: asyncio.Task | None = None
+    settings = get_settings()
+    if settings.sac_host_silence_scan_enabled:
+        from app.jobs.host_silence_background import host_silence_scan_loop
+
+        scan_task = asyncio.create_task(host_silence_scan_loop(stop_scan))
+
     yield
+
+    if scan_task is not None:
+        stop_scan.set()
+        scan_task.cancel()
+        with suppress(asyncio.CancelledError):
+            await scan_task
     logger.info("%s — application shutdown", APP_VERSION_LABEL)