feat(distribution): 补齐退款加价回退、本站自卖与开通默认规则

退款时压缩卖方加价，人工/自动批次仅完结单可入账，号主收入进主站钱包；仲裁后重算释放并支持作废未入账 DS。开通时四种模式按默认生效，对外口径合并为 35/40/25，不再单独展示通道费。

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

backend/internal/modules/distribution/pricing_test.go

@@ -10,6 +10,7 @@ func TestDefaultRules(t *testing.T) {
 		{ModeTwoPartyDistributorReceive, func(r Rule) bool { return r.DistributorReceiveRatio == 35 && r.PlatformSellRatio == 40 }},
 		{ModeTwoPartyPlatformReceive, func(r Rule) bool { return r.PlatformReceiveRatio == 35 && r.DistributorSellRatio == 40 }},
 		{ModeThreeParty, func(r Rule) bool { return r.DistributorARatio == 35 && r.DistributorBRatio == 40 }},
+		{ModeSiteSelfSell, func(r Rule) bool { return r.DistributorSellRatio == 75 && r.ServiceFeeValue == 25 }},
 	}
 	for _, item := range cases {
 		rule, err := DefaultRule(item.mode)
@@ -17,8 +18,8 @@ func TestDefaultRules(t *testing.T) {
 			t.Fatalf("default rule %s = %#v, err=%v", item.mode, rule, err)
 		}
 	}
-	if _, err := DefaultRule(ModeSiteSelfSell); err != ErrUnsupportedMode {
-		t.Fatalf("site self-sell must not have a default split, err=%v", err)
+	if _, err := DefaultRule("unknown_mode"); err != ErrUnsupportedMode {
+		t.Fatalf("unknown mode must be unsupported, err=%v", err)
 	}
 }
 
@@ -116,3 +117,54 @@ func TestSplitSettlementThreeParty(t *testing.T) {
 		t.Fatalf("splits = %#v", splits)
 	}
 }
+
+func TestCalculateSettlementClampsMarkupToRemainingNetPaid(t *testing.T) {
+	rule := Rule{ServiceFeeValue: 25, ServiceFeeUnit: ServiceFeeUnitPercent, PaymentFeeBPS: 50}
+	got, err := CalculateSettlement(rule, SettlementInput{
+		GrossPaidAmountCent: 10000, RefundAmountCent: 10000, OwnerRentAmountCent: 6000, MarkupAmountCent: 3000,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.MarkupAmountCent != 0 || got.SplitBaseAmountCent != 0 || got.NetPaidAmountCent != 0 {
+		t.Fatalf("full refund must zero markup and profit: %#v", got)
+	}
+
+	partial, err := CalculateSettlement(rule, SettlementInput{
+		GrossPaidAmountCent: 10000, RefundAmountCent: 2000, OwnerRentAmountCent: 6000, MarkupAmountCent: 3000,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if partial.NetPaidAmountCent != 8000 || partial.MarkupAmountCent != 2000 || partial.SplitBaseAmountCent != 0 {
+		t.Fatalf("partial refund should clamp markup to remainder: %#v", partial)
+	}
+}
+
+func TestSplitSettlementSiteSelfSell(t *testing.T) {
+	rule, err := DefaultRule(ModeSiteSelfSell)
+	if err != nil {
+		t.Fatal(err)
+	}
+	splits := SplitSettlement(rule, 10000, 2000)
+	if len(splits) != 1 || splits[0].PartyRole != "distributor_sell" || splits[0].AmountCent != 9500 {
+		t.Fatalf("self-sell split = %#v, want 7500+2000", splits)
+	}
+}
+
+func TestValidateRuleRequiresPercentSum(t *testing.T) {
+	rule, err := DefaultRule(ModeTwoPartyDistributorReceive)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := ValidateRule(rule); err != nil {
+		t.Fatalf("default rule should be valid: %v", err)
+	}
+	rule.DistributorReceiveRatio = 10
+	if err := ValidateRule(rule); err != ErrInvalidRate {
+		t.Fatalf("invalid percent sum err = %v", err)
+	}
+	if err := ValidateRule(Rule{Mode: "not_a_mode", ServiceFeeUnit: ServiceFeeUnitPercent}); err != ErrUnsupportedMode {
+		t.Fatalf("unsupported mode err = %v", err)
+	}
+}