feat(receivables): customer-level rows only + register hanke_ar as balance snapshot batch

ASK-20261001-33: api_receivables counts only rows with a non-blank counterparty (monthly aggregate rows stay in table but out of the collection page cards/aging/detail/owners); BALANCE_SNAPSHOT_SOURCES gains hanke_ar (default skip, no per-customer overdue alerts). New pytest file covers aggregate-row exclusion, e2 baseline digitwise, snapshot registration.
(cherry picked from commit 475f78bc8c5f67f2716c721172a88ff89ec58227)

backend/app/utils/cash_forecast_engine.py

@@ -55,13 +55,17 @@ KPI_CODE_SUFFIXES = {
 # ── 余额快照批次（F4，2026-09-13）────────────────────────────
 # 这些 source 是「期末余额导入」批次：plan_date 是**导入基准日**（如 2026-06-30 期末），
 # 不是真实到期日 → 逐户生成"到期未收款"会永远 overdue，属伪逾期（DAMA 审计 fin-alert-flood-e2）。
+# hanke_ar（酣客 2026-07 期末客户级应收，ASK-20261001-33）语义与 bohai_ar_20260630 完全相同，
+# 2026-10-01 按设计件 §6.2「快照批次登记」加入名单：默认 skip ⇒ 不逐户生成伪逾期告警。
 BALANCE_SNAPSHOT_SOURCES = {
     "bohai_ar_20260630",
     "bohai_ap_20260630",
     "bohai_prepay_20260630",
     "receivables_migrate",
+    "hanke_ar",
 }
 
+
 # 处理开关：per_customer(逐户，旧行为) / skip(不生成逐户，默认) / aggregate(按批次聚合1条)
 _SNAPSHOT_MODE_DEFAULT = "skip"
 _SNAPSHOT_MODES = ("per_customer", "skip", "aggregate")