fix(bep): pick target period by period-closed signal, sales threshold as fallback only

Defect: _pick_target_period treated "month sales < 30% of 6-month avg" as
"period not closed", so 2026-08 (actually closed: 3103 carry-forward vouchers
present) was skipped because of a low-sales off-season month, pinning the five
F_BEP* KPIs at 2026-07.

Fix: add _is_period_closed() - primary signal voucher_details
(carry_forward=1 AND subject_code='3103'), secondary signal
ZXT_LANDING_DIR/{YYYY.MM}/check.md containing both "issue PASS" and
"P&L carry-forward PASS". _pick_target_period now scans candidate months desc
and returns the first closed one, with no sales filtering; the old sales
threshold path is kept only as a fallback (warns "sales-threshold fallback").

Scope: run_derive() formulas, collector, kpi_definitions and cron untouched.

Verified: period=2026-08; 167.5 / 60.4 / 39.6 / 118.7 / 85.2;
F_BEP_COVER 2026-08 red pending, 2026-07 pending resolved; idempotent on re-run.

backend/scripts/kpi_bep_deriver.py

@@ -13,7 +13,9 @@
 - 折旧:     subject_code = '560213'(管理费用-折旧叶子) 借方合计，YTD 月均 → 现金口径剔除项(eq精确匹配,防5602130误含)
 - 渠补率:   厂家政策定死 84.61% (571103渠补支出/5001销售, 1-7月累计凭证验证≈84.0%~84.6%)
 - 边际贡献率 = 1 - 渠补率 = 15.39%
-- 目标月:   --period 显式指定；默认自动选最近完整月（当月跳过；最近月销售额<近6月均30%视为未结账回退）
+- 目标月:   --period 显式指定；默认自动选最近「已结账」完整月（当月跳过）。
+            结账信号优先：DB 损益结转凭证(carry_forward=1 & subject_code='3103') → check.md 文件；
+            两者皆无时才退回旧「销售额阈值」兜底（销售低 ≠ 未结账，仅保底用，日志标注依据）
 - 单位:     voucher 金额为元，聚合后 /10000 → 万元
 - 幂等:     同 kpi_id+period+source_type='derived' 存在则更新，可安全重跑
 - 只写 source_type='derived' 行；不触碰 auto_collect/manual/excel/ledger 行；不改 collector 逻辑
@@ -59,6 +61,10 @@ SALES_CODE = "5001"          # 主营业务收入
 ADMIN_PREFIX = "5602"        # 管理费用（含折旧叶子560213）
 DEP_SUBJECT = "560213"       # 管理费用-折旧（现金口径剔除项，eq 精确匹配防 5602130 误含）
 REBATE_CODE = "571103"       # 渠补/捐赠支出（校验用，不直接进公式）
+CARRY_SUBJECT = "3103"       # 本年利润（carry_forward=1 且该科目 = 该期损益结转凭证）
+
+# 智享通落盘目录（与 app/api/data.py 的 ZXT_LANDING_DIR 一致）——结账信号次选来源
+ZXT_LANDING_DIR = "/root/WB_WS/_data/zxt-hanke"
 
 
 def _month_range(period: str):
@@ -101,8 +107,49 @@ def _month_sales(db, entity_id: int, period: str) -> float:
                       ("eq", SALES_CODE)) / 10000.0
 
 
+def _is_period_closed(db, entity_id: int, period: str) -> tuple:
+    """判断该期间是否已结账（损益结转已完成）。返回 (bool, 依据说明)。
+
+    主信号（DB）: voucher_details 存在 carry_forward=1 且 subject_code='3103'(本年利润) 的行
+                  = 该期损益结转凭证已生成
+    次信号（文件）: ZXT_LANDING_DIR/{YYYY.MM}/check.md 同时含「本期: PASS」与「损益结转: PASS」
+    均不成立 → (False, "")。任何读取异常均吞掉，不影响主流程。
+    """
+    # 主信号：DB 损益结转凭证
+    try:
+        hit = db.query(VoucherDetail.id).filter(
+            VoucherDetail.entity_id == entity_id,
+            VoucherDetail.carry_forward == 1,
+            VoucherDetail.subject_code == CARRY_SUBJECT,
+            VoucherDetail.period == period,
+        ).first()
+        if hit:
+            return True, "DB损益结转"
+    except Exception as e:  # 查询异常不阻断，降级到文件信号
+        logger.warning("结账信号(DB)查询失败 period=%s: %s", period, e)
+
+    # 次信号：智享通落盘 check.md
+    try:
+        path = os.path.join(ZXT_LANDING_DIR, period.replace("-", "."), "check.md")
+        if os.path.exists(path):
+            with open(path, "r", encoding="utf-8") as f:
+                txt = f.read()
+            if "本期: PASS" in txt and "损益结转: PASS" in txt:
+                return True, "check.md"
+    except Exception:
+        pass  # 文件缺失/读取失败一律忽略
+
+    return False, ""
+
+
 def _pick_target_period(db, entity_id: int, now: date) -> str:
-    """自动选择目标月：跳过当月；最近月销售额<近6月完整月均30%则视为未结账回退"""
+    """自动选择目标月：跳过当月。
+
+    判定顺序（2026-09-12 修订）:
+      1. 结账信号优先 —— 按 desc 返回第一个 _is_period_closed 为真的月；
+         结账成立时不再用销售额做任何筛选（销售低 ≠ 未结账，历史缺陷根因）
+      2. 所有候选月均无结账信号 → 退回旧「销售额 < 近6月均×30%」兜底（日志标注『依据销售额兜底』）
+    """
     # 收集已有数据月份（5001 贷方）
     rows = db.query(VoucherDetail.period).filter(
         VoucherDetail.entity_id == entity_id,
@@ -121,6 +168,17 @@ def _pick_target_period(db, entity_id: int, now: date) -> str:
     if not candidates:
         candidates = periods
 
+    # 1) 结账信号优先（DB损益结转 → check.md），命中即采用
+    for p in candidates:
+        closed, reason = _is_period_closed(db, entity_id, p)
+        if closed:
+            logger.info("目标月 %s 判定已结账（依据：%s），采用", p, reason)
+            return p
+
+    # 2) 兜底：无任何结账信号时，退回旧销售额阈值逻辑
+    logger.warning("候选月 %s 均无结账信号(DB损益结转/check.md)，退回销售额阈值兜底",
+                   ", ".join(candidates[:6]))
+
     # 参考月均：候选月中最近 6 个不同月的月销售
     ref_months = []
     for p in periods:
@@ -137,8 +195,10 @@ def _pick_target_period(db, entity_id: int, now: date) -> str:
         if s <= 0:
             continue  # 该月无销售，往前找
         if ref_avg and s < ref_avg * 0.3:
-            logger.info("目标月 %s 销售 %.1f万 < 参考月均 %.1f万*30%%，视为未结账，回退", p, s, ref_avg)
+            logger.info("目标月 %s 销售 %.1f万 < 参考月均 %.1f万*30%%，依据销售额兜底判定未结账，回退",
+                        p, s, ref_avg)
             continue
+        logger.info("目标月 %s 依据销售额兜底采用", p)
         return p
     return candidates[0]