[阶段1.1-1.7] 实现完整的认证系统

后端实现:
- 创建 AuthService 包含密码加密、JWT 生成和验证
- 创建 authMiddleware 用于 API 路由保护
- 实现 auth 路由 (register, login, logout, /me)

前端实现:
- 创建 Login 页面 (登录/注册标签页)
- 创建 ProtectedRoute 组件用于路由保护
- 更新 App.tsx 集成路由保护
- 前端 API 客户端已包含认证方法和拦截器

验收标准已满足:
- 用户可以注册和登录
- JWT Token 正确生成和验证
- 受保护的路由需要有效 Token
- 未认证用户重定向到登录页面

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/auth.py

@@ -0,0 +1,90 @@
+"""
+Authentication helpers: scrypt password hashing, JWT signing/verification,
+and the require_auth decorator used by route blueprints.
+
+Password hashing mirrors the original Node implementation exactly:
+    salt (16 random bytes, hex) : scrypt(password, salt, n=16384, r=8, p=1, dklen=64, hex)
+"""
+import hashlib
+import hmac
+import os
+import datetime
+
+import jwt
+from flask import request, g, jsonify
+from functools import wraps
+
+from config import JWT_SECRET, JWT_EXPIRY_DAYS
+
+
+class AuthError(Exception):
+    def __init__(self, code, message):
+        super().__init__(message)
+        self.code = code
+        self.message = message
+
+
+def hash_password(password: str) -> str:
+    salt = os.urandom(16)
+    derived = hashlib.scrypt(
+        password.encode("utf-8"),
+        salt=salt,
+        n=16384,
+        r=8,
+        p=1,
+        dklen=64,
+    )
+    return f"{salt.hex()}:{derived.hex()}"
+
+
+def verify_password(password: str, stored: str) -> bool:
+    if not stored or ":" not in stored:
+        return False
+    salt_hex, hash_hex = stored.split(":", 1)
+    try:
+        salt = bytes.fromhex(salt_hex)
+    except ValueError:
+        return False
+    derived = hashlib.scrypt(
+        password.encode("utf-8"),
+        salt=salt,
+        n=16384,
+        r=8,
+        p=1,
+        dklen=64,
+    )
+    return hmac.compare_digest(derived.hex(), hash_hex)
+
+
+def sign_token(user_id: str) -> str:
+    now = datetime.datetime.utcnow()
+    payload = {
+        "sub": user_id,
+        "iat": now,
+        "exp": now + datetime.timedelta(days=JWT_EXPIRY_DAYS),
+    }
+    return jwt.encode(payload, JWT_SECRET, algorithm="HS256")
+
+
+def verify_token(token: str) -> dict:
+    payload = jwt.decode(token, JWT_SECRET, algorithms=["HS256"])
+    return {"user_id": payload["sub"]}
+
+
+def require_auth(f):
+    @wraps(f)
+    def wrapper(*args, **kwargs):
+        auth = request.headers.get("Authorization", "")
+        if not auth.startswith("Bearer "):
+            return jsonify({"error": "missing or malformed Authorization header"}), 401
+        token = auth[7:].strip()
+        try:
+            data = verify_token(token)
+        except jwt.ExpiredSignatureError:
+            return jsonify({"error": "token expired"}), 401
+        except jwt.InvalidTokenError:
+            return jsonify({"error": "invalid token"}), 401
+        g.user_id = data["user_id"]
+        return f(*args, **kwargs)
+
+    return wrapper