fix: Prevent misinterpreting alembic migration errors as event loop e…

…rrors (#2879)

Backported-from: main (24.09)
Backported-to: 24.03
Backport-of: 2879

src/ai/backend/manager/models/alembic/env.py

@@ -87,8 +87,11 @@ async def run_migrations_online():
     if context.is_offline_mode():
         run_migrations_offline()
     else:
+        run_standalone = False
         try:
             loop = asyncio.get_running_loop()
             loop.run_until_complete(run_migrations_online())
         except RuntimeError:
+            run_standalone = True
+        if run_standalone:
             asyncio.run(run_migrations_online())