feat: 账套模式后端 — token绑定entity_id + user_entities授权表 + 解析链倒置

- auth_middleware: create_token存JSON{user_id,entity_id}，旧int格式token强制下线
- models: 新增UserEntity授权表(user_id↔entity_id多对多,唯一约束)
- database: init_db自动建表+存量用户×active企业默认授权(平滑迁移)
- deps: get_entity_id解析链倒置 token优先 → Bot白名单(query/header校验entity active) → 默认1
- auth: login加entity_id+授权校验; 新增switch-entity/my-entities/登录页entities接口; register自动授权

backend/app/models/__init__.py

@@ -1,5 +1,5 @@
 """管理会计OS 数据模型"""
-from sqlalchemy import Column, Integer, String, Text, Float, DateTime, ForeignKey, Boolean, JSON, func
+from sqlalchemy import Column, Integer, String, Text, Float, DateTime, ForeignKey, Boolean, JSON, func, UniqueConstraint
 from app.database import Base
 
 from app.models.budget_plan import BudgetPlan
@@ -32,6 +32,17 @@ class User(Base):
     created_at = Column(DateTime, server_default=func.now())
 
 
+class UserEntity(Base):
+    """用户-企业授权（账套模式多对多）— 新表必须带entity_id（开发规范）"""
+    __tablename__ = "user_entities"
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False, index=True, comment="用户ID")
+    entity_id = Column(Integer, ForeignKey("entities.id"), nullable=False, index=True, comment="企业ID（账套）")
+    granted_by = Column(Integer, nullable=True, comment="授权人")
+    created_at = Column(DateTime, server_default=func.now())
+    __table_args__ = (UniqueConstraint("user_id", "entity_id", name="uq_user_entity"),)
+
+
 class StrategicMap(Base):
     """战略地图"""
     __tablename__ = "strategic_maps"