Replace passlib with direct bcrypt calls, drop passlib dependency

passlib is unmaintained and incompatible with bcrypt 5.0.0 due to its
internal wrap-bug detection test exceeding bcrypt's 72-byte password limit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/auth.py

@@ -2,8 +2,8 @@ import os
 from datetime import datetime, timedelta, timezone
 from typing import Optional
 
+import bcrypt
 from jose import JWTError, jwt
-from passlib.context import CryptContext
 from fastapi import Cookie, Depends, HTTPException, Response, status
 from sqlalchemy.orm import Session
 
@@ -16,15 +16,12 @@ ACCESS_TOKEN_EXPIRE_DAYS = 30
 COOKIE_NAME = "token"
 SECURE_COOKIES = os.environ.get("SECURE_COOKIES", "true").lower() == "true"
 
-pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
-
-
 def verify_password(plain_password: str, hashed_password: str) -> bool:
-    return pwd_context.verify(plain_password, hashed_password)
+    return bcrypt.checkpw(plain_password.encode(), hashed_password.encode())
 
 
 def hash_password(password: str) -> str:
-    return pwd_context.hash(password)
+    return bcrypt.hashpw(password.encode(), bcrypt.gensalt()).decode()
 
 
 def create_access_token(user_id: int, username: str, is_admin: bool, user_timezone: str = "UTC", admin_id: Optional[int] = None) -> str: