refactor: 权限组简化为2组(readonly/write) + Transfer穿梭框选择数据库

- access_type 从3值(readonly/datachange/ddl)简化为2值(readonly/write)
- readonly=只读查看(关联只读/从库用于查询) write=写入变更(关联主库用于工单)
- 前端InstanceGroupPage重写: 2权限类型卡片+自定义Transfer穿梭框组件
- Transfer组件: 左侧可选数据库→右侧已授权数据库, 搜索/勾选/箭头移动
- role_permissions.py更新: 2值权限映射, get_accessible_instances按access_type过滤
- 后端schema/模型校验更新为readonly|write正则
- 15个测试全部通过

backend/app/models/instance_group.py

@@ -26,7 +26,7 @@ class InstanceGroup(Base):
     description = Column(Text, nullable=True)
     db_type = Column(String(20), nullable=False, default="all")  # mysql/redis/pg/mongo/all
     permissions = Column(Text, nullable=True)  # JSON: ["query:select", ...] (legacy, kept for compat)
-    access_type = Column(String(20), nullable=True, default="readonly")  # readonly / datachange / ddl
+    access_type = Column(String(20), nullable=True, default="readonly")  # readonly / write
     allowed_databases = Column(Text, nullable=True)  # JSON array of database names; empty = all databases
     is_active = Column(Boolean, default=True)
     created_at = Column(DateTime, server_default=func.now())
@@ -65,4 +65,4 @@ class InstanceGroup(Base):
             json.dumps(databases, ensure_ascii=False) if databases else None
         )
 
-    VALID_ACCESS_TYPES = {"readonly", "datachange", "ddl"}
+    VALID_ACCESS_TYPES = {"readonly", "write"}