test: add tests for status WebSocket and scheduler

- test_status.py: 8 tests covering WS auth rejection/acceptance, broadcast_status (dead connection removal, no response_time, no connections), broadcast_scan_update
- test_scheduler.py: 9 tests covering _load_interval variants, _run_status_checks DB update/last_seen/error handling, start/stop lifecycle
- scheduler.py: reinitialize AsyncIOScheduler on each start() to avoid stale event loop across test restarts

backend/app/core/scheduler.py

@@ -13,7 +13,7 @@ from app.services.status_checker import check_node
 
 logger = logging.getLogger(__name__)
 
-scheduler = AsyncIOScheduler()
+scheduler: AsyncIOScheduler = AsyncIOScheduler()
 
 
 async def _run_status_checks() -> None:
@@ -56,6 +56,8 @@ def _load_interval() -> int:
 
 
 def start_scheduler() -> None:
+    global scheduler
+    scheduler = AsyncIOScheduler()
     interval = _load_interval()
     scheduler.add_job(_run_status_checks, "interval", seconds=interval, id="status_checks")
     scheduler.start()