feat(system): 计提参数版本管理、变更留痕与计算预览接口

- 9 个端点：目录快照(asOf)/单参数历史/变更批次提交与撤回/批次分页与明细/两日期对比/Excel 导出/计算预览
- submit_change 全量前置校验（注册表/维度岗位/取值域/同批去重）→ 建批次 → 逐项留痕（旧值取提交时生效值）
- 撤回：FOR UPDATE 锁行 + 已生效禁撤，删除后解析自然回退；同日多次变更取最新留痕
- 预览按岗位模板分派公式（经理聚合口径明确拒绝），出参含所用参数与生效时点；导出走 openpyxl
- ECharts 决策记录：前端统计图使用 echarts（用户指定）

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

backend/app/api/router.py

@@ -10,6 +10,7 @@ from app.api import health
 from app.api.v1 import auth
 from app.api.v1.system import business_roles as system_business_roles
 from app.api.v1.system import logs as system_logs
+from app.api.v1.system import params as system_params
 from app.api.v1.system import roles as system_roles
 from app.api.v1.system import users as system_users
 
@@ -27,6 +28,7 @@ system_router = APIRouter(prefix="/system")
 system_router.include_router(system_users.router)
 system_router.include_router(system_roles.router)
 system_router.include_router(system_business_roles.router)
+system_router.include_router(system_params.router)
 system_router.include_router(system_logs.operation_router)
 system_router.include_router(system_logs.login_router)
 v1_router.include_router(system_router)