feat(phase2): shared/ 共享层迁移

- shared/: enums/constants/pagination/base_model/base_db/base_repository/base_service
- apps/constants 与 apps/models/base_model、base_db 改为垫片
- 新增 pagination 单元测试

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

shared/__init__.py

@@ -0,0 +1,17 @@
+# +======================================================================
+# | DjangoAdmin 企业数字化开发平台 [ 赋能开发者，助力企业发展 ]
+# +----------------------------------------------------------------------
+# | Copyright (c) 2021-2026 一米阳光团队 版权所有
+# | 开源协议: Apache-2.0
+# +======================================================================
+
+"""跨模块共享层：枚举、常量、分页、模型基类、Repository 基类。"""
+
+from shared.constants import (  # noqa: F401
+    PAGE_SIZE,
+    DEFAULT_PASSWORD,
+    MESSAGE_OK,
+    MESSAGE_FAIL,
+    SERIALIZE_EXCLUDE_FIELDS,
+)
+from shared.enums import *  # noqa: F401,F403