fix: use add_startup() for cache_sync instead of asyncio.create_task

- init() is called synchronously before the event loop starts
  (configuredServer.py:36), so asyncio.create_task() fails with
  'no running event loop'
- Use ahserver's add_startup() which appends to app.on_startup
  and runs after the loop is ready (configuredServer.py:118)
- Remove unused asyncio import

app/sage.py

@@ -1,6 +1,5 @@
 import os, sys
 import json
-import asyncio
 import argparse
 from appPublic.log import MyLogger, info, debug, warning
 from appPublic.folderUtils import ProgramPath
@@ -10,7 +9,7 @@ from appPublic.registerfunction import RegisterFunction
 from bricks_for_python.init import load_pybricks
 from ahserver.webapp import webapp
 from ahserver.serverenv import ServerEnv
-from ahserver.configuredServer import add_cleanupctx
+from ahserver.configuredServer import add_cleanupctx, add_startup
 from rbac.init import load_rbac, start_cache_sync
 from pricing.init import load_pricing
 from appbase.init import load_appbase
@@ -42,7 +41,7 @@ def init():
 	load_pybricks()
 	load_appbase()
 	load_rbac()
-	asyncio.create_task(start_cache_sync())
+	add_startup(start_cache_sync)
 	load_accounting()
 	load_unipay()
 	load_platformbiz()