feat(crm): add settlement clause default text helper

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/crm_settlement_clause.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+from typing import Optional
+
+ADVANCE_PAYMENT = "advance_payment"
+ARRIVE_PAYMENT = "arrive_payment"
+
+CLAUSE_LABELS = {
+    "volume_settlement": "按量结算",
+    "monthly_settlement": "月结",
+    "quarterly_settlement": "季结",
+    "other_settlement": "其他",
+}
+
+ADVANCE_CLAUSE = (
+    "款到发货。需方付款后安排发货，需方收到货物后，供方开具13%增值税专用发票。"
+)
+ARRIVE_CLAUSE = (
+    "货到付款，需方收到供方货物后，供方开具13%增值税专用发票，无异议后，以现汇方式月结。"
+)
+
+
+def default_settlement_clause(settlement_method: Optional[str]) -> str:
+    method = (settlement_method or "").strip() or ARRIVE_PAYMENT
+    if method == ADVANCE_PAYMENT:
+        return ADVANCE_CLAUSE
+    if method in CLAUSE_LABELS:
+        label = CLAUSE_LABELS[method]
+        return (
+            f"请根据双方约定填写结算方式：{label}。"
+            "供方开具13%增值税专用发票，无异议后，以现汇方式月结。"
+        )
+    # arrive_payment 与未知值
+    return ARRIVE_CLAUSE

backend/tests/test_crm_settlement_clause.py

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+from app.services.crm_settlement_clause import default_settlement_clause
+
+
+def test_advance_payment_clause():
+    assert "款到发货" in default_settlement_clause("advance_payment")
+
+
+def test_arrive_payment_clause():
+    assert "货到付款" in default_settlement_clause("arrive_payment")
+
+
+def test_monthly_settlement_clause_uses_label():
+    text = default_settlement_clause("monthly_settlement")
+    assert text.startswith("请根据双方约定填写结算方式：月结。")
+    assert "13%增值税专用发票" in text
+
+
+def test_volume_quarterly_other_labels():
+    assert "按量结算" in default_settlement_clause("volume_settlement")
+    assert "季结" in default_settlement_clause("quarterly_settlement")
+    assert "其他" in default_settlement_clause("other_settlement")
+
+
+def test_unknown_falls_back_to_arrive():
+    assert default_settlement_clause("weird") == default_settlement_clause("arrive_payment")
+
+
+def test_empty_falls_back_to_arrive():
+    assert default_settlement_clause("") == default_settlement_clause("arrive_payment")
+    assert default_settlement_clause(None) == default_settlement_clause("arrive_payment")