feat(calc): 单笔回款核算引擎与绩效台账

- calc_service.calc_receipt：锁回款行→settlement_status=0 校验→发货批次/
  归属前置→resolve_snapshot(回款日)→按 calc_template 分派（与 _preview_rate
  同构，‰÷1000 防线）→items 行级折算（行类别阈值，混合按行汇总）→
  barter/起付/R→台账 v1→CAS 置已核算，单 audit 事务
- 策略点集中：_personal_rate（#12 经理个人户单笔落 0 待答复）、_row_profit
  （#20 不含税原样）；核算粒度 #25 一期口径=业务级一次性计提（重复回款
  repeated=True 仅登记进度）
- 台账仓储：生效行查询/业务级判定/版本号；schemas LedgerOut/Query
- api /api/v1/calc/*：台账分页/详情/核算触发（写限 admin+marketing_ops）
- 测试 8 例：金标准双类别复算到分（DB 锚定 4000.00/39.11/39.11）、
  一次性计提、已核算 400、未指定批次、成本缺失行保守 -3000、负绩效照算、
  准入、列表筛选；行级明细折算两位固化
- conftest 清理顺序修正：V7 台账/月结清理移到回款/人员删除之前
  （RESTRICT 外键不挡序）

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

backend/app/api/router.py

@@ -15,6 +15,7 @@ from app.api.v1.business import businesses as business_businesses
 from app.api.v1.business import ratings as business_ratings
 from app.api.v1.business import receipts as business_receipts
 from app.api.v1.business import shipments as business_shipments
+from app.api.v1.calc import router as calc_router
 from app.api.v1.system import approvals as system_approvals
 from app.api.v1.system import business_roles as system_business_roles
 from app.api.v1.system import logs as system_logs
@@ -59,4 +60,7 @@ business_router.include_router(business_receipts.router)
 business_router.include_router(business_ratings.router)
 v1_router.include_router(business_router)
 
+# 绩效核算（M3 ④）：/api/v1/calc/*，读登录即可、写限管理员与营销后勤
+v1_router.include_router(calc_router)
+
 api_router.include_router(v1_router)