Standardize async offloading behind shared executor helper

backend/app/services/auth_service.py

@@ -8,6 +8,7 @@ survive server restarts.
 from __future__ import annotations
 
 import asyncio
+from app.utils.async_utils import run_blocking
 import hashlib
 import hmac
 import secrets
@@ -72,9 +73,7 @@ async def _check_password(plain: str, hashed: str) -> bool:
     plain_bytes = plain.encode()
     hashed_bytes = hashed.encode()
     loop = asyncio.get_running_loop()
-    return await loop.run_in_executor(
-        None, lambda: bool(bcrypt.checkpw(plain_bytes, hashed_bytes))
-    )
+    return await run_blocking(lambda: bool(bcrypt.checkpw(plain_bytes, hashed_bytes)))
 
 
 async def login(