feat(settlement): 月度结算快照生成与审批锁定，调整单（补发/扣减）与审批联动

- settlement_service：generate（确认回款口径 #8 策略点=有效台账行按回款日
  归月、经理聚合 #12 暂口径基数不含经理个人户、月结 uk 一行一月、草稿/
  驳回可重生成、已审定/待审 400）→ submit（审批开关分流，关=自动通过即
  锁定、开=待财务复核）→ approval_service.act 扩 order_type=2/3 钩子
  （通过=mark_locked/调整单生效；驳回=回草稿/作废，延迟导入防循环）
- 锁定拦截：回款新增写路径校验回款月已审定 400（比需求「提示」取硬，
  一期安全侧；M2 已核算禁改防线先于其生效）
- 调整单：新建（TZ 单号，开关分流自动通过留痕/走审批）+ 列表
- schemas settlement：金额 Decimal 声明（str 声明遇 Decimal 入参拒绝）
- 测试 6 例：生成回填/无记录 400/已审定重生成 400/关自动锁定/锁定后回款
  禁改/开两节点流转至锁定（finance→leader）

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

backend/app/api/router.py

@@ -16,6 +16,7 @@ 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.settlements import router as settlements_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
@@ -62,5 +63,7 @@ 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)
+# 月度结算（M3 ④）：/api/v1/settlements/*，读登录即可、写限管理员与营销后勤
+v1_router.include_router(settlements_router)
 
 api_router.include_router(v1_router)