feat: 新增后端支付模块，支持微信和支付宝

- 新增api/payment/支付服务（微信、支付宝）
- 新增api/routers/payment.py支付路由
- 更新database/models.py支付相关模型
- 新增数据库迁移文件（订单表、用户订阅字段）
- 更新main.py、requirements.txt、.env.production

Co-authored-by: Cursor <cursoragent@cursor.com>

api/database/models.py

@@ -27,12 +27,14 @@ class User(Base):
     nickname = Column(String, nullable=False)
     avatar_url = Column(String, nullable=True)
     subscription_type = Column(String, default="free")  # free, premium
+    subscription_expires_at = Column(DateTime(timezone=True), nullable=True)  # 订阅到期时间
     created_at = Column(DateTime(timezone=True), default=utc_now)
 
     # Relationships
     conversations = relationship("Conversation", back_populates="user")
     chapters = relationship("Chapter", back_populates="user")
     books = relationship("Book", back_populates="user")
+    orders = relationship("Order", back_populates="user", cascade="all, delete-orphan")
     memoir_state = relationship("MemoirState", back_populates="user", uselist=False, cascade="all, delete-orphan")
     refresh_tokens = relationship("RefreshToken", back_populates="user", cascade="all, delete-orphan")
 
@@ -158,3 +160,24 @@ class SmsVerificationCode(Base):
     verified_at = Column(DateTime(timezone=True), nullable=True)  # 验证时间
     ip_address = Column(String, nullable=True)  # 请求IP地址
 
+
+class Order(Base):
+    """支付订单表"""
+    __tablename__ = "orders"
+
+    id = Column(String, primary_key=True)  # 内部订单号
+    user_id = Column(String, ForeignKey("users.id"), nullable=False, index=True)
+    plan_id = Column(String, nullable=False)  # 套餐 ID（free / premium）
+    plan_name = Column(String, nullable=False)  # 套餐名称
+    amount = Column(Integer, nullable=False)  # 金额（单位：分）
+    currency = Column(String, default="CNY")
+    payment_method = Column(String, nullable=False)  # wechat / alipay
+    status = Column(String, default="pending")  # pending / paid / failed / cancelled / refunded
+    trade_no = Column(String, nullable=True, index=True)  # 第三方交易号（微信/支付宝）
+    paid_at = Column(DateTime(timezone=True), nullable=True)  # 支付完成时间
+    created_at = Column(DateTime(timezone=True), default=utc_now)
+    expired_at = Column(DateTime(timezone=True), nullable=True)  # 订单超时时间
+
+    # Relationships
+    user = relationship("User", back_populates="orders")
+