feat(base): 新增基础数据六表及 V4 迁移（人员/辐射区域/拓展计划/产品/客户/归属留痕）

- TPlusSyncMixin 统一 T+ 同步预留四列（可空+普通索引，唯一键待 IT 定键后升级）
- 留痕表实体 ID 无外键冗余固化，删除保护由服务层承担
- init-dev.sql 同步六表 DDL，alembic_version 标记更新

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

backend/app/models/__init__.py

@@ -11,21 +11,30 @@
 
 from app.models.base import Base, TimestampMixin
 from app.models.business_role import BusinessRole
+from app.models.customer import Customer, OwnershipChangeLog
 from app.models.login_log import LoginLog
 from app.models.operation_log import OperationLog
 from app.models.param import ParamChangeBatch, ParamDefinition, ParamValue
+from app.models.personnel import Personnel, PersonnelPlan, PersonnelRegion
+from app.models.product import Product
 from app.models.role import Permission, Role, RolePermission
 from app.models.user import User, UserRole
 
 __all__ = [
     "Base",
     "BusinessRole",
+    "Customer",
     "LoginLog",
     "OperationLog",
+    "OwnershipChangeLog",
     "ParamChangeBatch",
     "ParamDefinition",
     "ParamValue",
     "Permission",
+    "Personnel",
+    "PersonnelPlan",
+    "PersonnelRegion",
+    "Product",
     "Role",
     "RolePermission",
     "TimestampMixin",