fix(503-1 · 503-2 로그인): 성공 뒤 세션 캐시 비움(되튕김) · 마무리 쓰기 한 트랜잭션 · bcrypt 스레드

- requestLogin 응답 뒤 clearSessionCache — 화면 뜬 뒤 4 s 안 누름도 대시보드
- finish_login = 한 커넥션 · begin 1 · commit 1(users UPDATE · 신뢰 기기 · 세션 · login_logs) · 실패 = rollback
- verify_password = asyncio.to_thread(로그인 · OTP 확인) — 이벤트 루프 안 막음
- 시험 test_309_1_login.py

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

common_util/common_util_auth.py

@@ -3,7 +3,7 @@
 import hashlib
 import re
 import secrets
-from datetime import datetime, timedelta
+from datetime import datetime
 from typing import Any
 
 import bcrypt
@@ -88,20 +88,6 @@ def delete_session_cookie(response: Response) -> None:
     )
 
 
-async def create_session(user_id: int, user_agent: str) -> str:
-    session_id = secrets.token_urlsafe(32)
-    expires_at = datetime.utcnow() + timedelta(seconds=SESSION_MAX_AGE_SECONDS)
-    pool = get_db_pool()
-    async with pool.acquire() as connection, connection.cursor() as cursor:
-        await cursor.execute(
-            """INSERT INTO sessions (id, user_id, user_agent_hash, expires_at)
-               VALUES (%s, %s, %s, %s)""",
-            (session_id, user_id, hash_user_agent(user_agent), expires_at),
-        )
-        await connection.commit()
-    return session_id
-
-
 async def verify_session(request: Request) -> dict[str, Any]:
     session_id = request.cookies.get(SESSION_COOKIE_NAME)
     if not session_id: