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/repositories/settlement_repository.py

@@ -0,0 +1,94 @@
+"""月度结算仓储：只读写数据并 flush；commit/rollback 在服务层。"""
+
+from sqlalchemy import func, select
+from sqlalchemy.orm import Session
+
+from app.models import MonthlySettlement, SettlementItem
+from app.schemas.settlement import SettlementQuery
+
+
+def get_by_id(session: Session, settlement_id: int) -> MonthlySettlement | None:
+    return session.get(MonthlySettlement, settlement_id)
+
+
+def get_by_month(session: Session, month: str) -> MonthlySettlement | None:
+    """按月份取月结行（settlement_month 全表唯一）。"""
+    return session.execute(
+        select(MonthlySettlement).where(MonthlySettlement.settlement_month == month)
+    ).scalar_one_or_none()
+
+
+def add(session: Session, settlement: MonthlySettlement) -> MonthlySettlement:
+    session.add(settlement)
+    session.flush()
+    return settlement
+
+
+def list_settlements(session: Session, query: SettlementQuery) -> list[MonthlySettlement]:
+    return list(
+        session.execute(
+            select(MonthlySettlement)
+            .where(*_filters(query))
+            .order_by(MonthlySettlement.settlement_month.desc())
+            .offset(query.offset)
+            .limit(query.page_size)
+        ).scalars()
+    )
+
+
+def count_settlements(session: Session, query: SettlementQuery) -> int:
+    return int(
+        session.execute(
+            select(func.count()).select_from(MonthlySettlement).where(*_filters(query))
+        ).scalar_one()
+    )
+
+
+def _filters(query: SettlementQuery) -> tuple:
+    filters = []
+    if query.status is not None:
+        filters.append(MonthlySettlement.status == query.status)
+    return tuple(filters)
+
+
+def list_items(session: Session, settlement_id: int) -> list[SettlementItem]:
+    return list(
+        session.execute(
+            select(SettlementItem)
+            .where(SettlementItem.settlement_id == settlement_id)
+            .order_by(SettlementItem.id)
+        ).scalars()
+    )
+
+
+def replace_items(session: Session, settlement_id: int, rows: list[SettlementItem]) -> None:
+    """全量替换明细（先删后 flush 再插，防御写法；不 commit）。"""
+    session.execute(delete_items_stmt(settlement_id))
+    session.flush()
+    for row in rows:
+        row.settlement_id = settlement_id
+        session.add(row)
+    session.flush()
+
+
+def delete_items_stmt(settlement_id: int):
+    from sqlalchemy import delete
+
+    return delete(SettlementItem).where(SettlementItem.settlement_id == settlement_id)
+
+
+def update_ledger_settlement_id(
+    session: Session, ledger_ids: list[int], settlement_id: int
+) -> None:
+    """月结生成时把纳入的台账行回填 settlement_id（append-only 的受控回填）。"""
+    from sqlalchemy import update
+
+    from app.models import PerformanceLedger
+
+    if ledger_ids:
+        session.execute(
+            update(PerformanceLedger)
+            .where(PerformanceLedger.id.in_(ledger_ids))
+            .values(settlement_id=settlement_id)
+        )
+        session.flush()