feat(system): 新增岗位角色与计提参数四表及 V3 迁移

- business_roles/param_definitions/param_change_batches/param_values 四表 + 种子（3 内置岗位/16 参数定义/19 版本条目/初始批次 P20260101-00）
- 双轨迁移：Alembic bfbda9c3191d + db/migration/V3 导出 + init-dev.sql 同步（schema_sync 校验通过）
- 参数值以 Fraction 可解析字符串存储（4/360 禁 0.011111）；企业户 2‰→3‰ 双种子支持按到账日回溯

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

backend/app/models/__init__.py

@@ -10,15 +10,21 @@
 """
 
 from app.models.base import Base, TimestampMixin
+from app.models.business_role import BusinessRole
 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.role import Permission, Role, RolePermission
 from app.models.user import User, UserRole
 
 __all__ = [
     "Base",
+    "BusinessRole",
     "LoginLog",
     "OperationLog",
+    "ParamChangeBatch",
+    "ParamDefinition",
+    "ParamValue",
     "Permission",
     "Role",
     "RolePermission",