feat: start cache_sync for cross-process cache invalidation

- sage.py: Import start_cache_sync from rbac.init
  - Call asyncio.create_task(start_cache_sync()) after load_rbac()
  - Starts Redis Pub/Sub subscription for cache invalidation messages

app/sage.py

@@ -11,7 +11,7 @@ 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 rbac.init import load_rbac
+from rbac.init import load_rbac, start_cache_sync
 from pricing.init import load_pricing
 from appbase.init import load_appbase
 from llmage.init import load_llmage
@@ -42,6 +42,7 @@ def init():
 	load_pybricks()
 	load_appbase()
 	load_rbac()
+	asyncio.create_task(start_cache_sync())
 	load_accounting()
 	load_unipay()
 	load_platformbiz()