fix(usage): subscription billing honours group rate multiplier

Subscription-mode billing was consuming quota at TotalCost (raw) instead of
ActualCost (TotalCost * RateMultiplier), so per-group rate multipliers —
including free subscriptions (multiplier = 0) — were silently ignored.
Switch the three subscription cost writes in buildUsageBillingCommand,
finalizePostUsageBilling, and the legacy postUsageBilling fallback to
ActualCost, and add a table-driven test covering 2x / 0.5x / free multipliers
plus a balance-mode regression check.

backend/internal/service/gateway_service.go

@@ -7317,8 +7317,10 @@ func postUsageBilling(ctx context.Context, p *postUsageBillingParams, deps *bill
 	cost := p.Cost
 
 	if p.IsSubscriptionBill {
-		if cost.TotalCost > 0 {
-			if err := deps.userSubRepo.IncrementUsage(billingCtx, p.Subscription.ID, cost.TotalCost); err != nil {
+		// Subscription usage tracked by ActualCost so group rate multiplier
+		// consumes the quota at the expected speed.
+		if cost.ActualCost > 0 {
+			if err := deps.userSubRepo.IncrementUsage(billingCtx, p.Subscription.ID, cost.ActualCost); err != nil {
 				slog.Error("increment subscription usage failed", "subscription_id", p.Subscription.ID, "error", err)
 			}
 		}
@@ -7417,9 +7419,13 @@ func buildUsageBillingCommand(requestID string, usageLog *UsageLog, p *postUsage
 		}
 	}
 
+	// Record subscription / balance cost using ActualCost so the group (and any
+	// user-specific) rate multiplier consumes subscription quota at the expected
+	// speed. TotalCost remains the raw (pre-multiplier) value; downstream guards
+	// on "> 0" still correctly skip free subscriptions (RateMultiplier == 0).
 	if p.IsSubscriptionBill && p.Subscription != nil && p.Cost.TotalCost > 0 {
 		cmd.SubscriptionID = &p.Subscription.ID
-		cmd.SubscriptionCost = p.Cost.TotalCost
+		cmd.SubscriptionCost = p.Cost.ActualCost
 	} else if p.Cost.ActualCost > 0 {
 		cmd.BalanceCost = p.Cost.ActualCost
 	}
@@ -7478,8 +7484,8 @@ func finalizePostUsageBilling(p *postUsageBillingParams, deps *billingDeps, resu
 	}
 
 	if p.IsSubscriptionBill {
-		if p.Cost.TotalCost > 0 && p.User != nil && p.APIKey != nil && p.APIKey.GroupID != nil {
-			deps.billingCacheService.QueueUpdateSubscriptionUsage(p.User.ID, *p.APIKey.GroupID, p.Cost.TotalCost)
+		if p.Cost.ActualCost > 0 && p.User != nil && p.APIKey != nil && p.APIKey.GroupID != nil {
+			deps.billingCacheService.QueueUpdateSubscriptionUsage(p.User.ID, *p.APIKey.GroupID, p.Cost.ActualCost)
 		}
 	} else if p.Cost.ActualCost > 0 && p.User != nil {
 		deps.billingCacheService.QueueDeductBalance(p.User.ID, p.Cost.ActualCost)

backend/internal/service/gateway_service_subscription_billing_test.go

@@ -0,0 +1,85 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+)
+
+// TestBuildUsageBillingCommand_SubscriptionAppliesRateMultiplier locks in the fix
+// that subscription-mode billing honours the group (and any user-specific) rate
+// multiplier — i.e. cmd.SubscriptionCost tracks ActualCost (= TotalCost *
+// RateMultiplier), not raw TotalCost.
+func TestBuildUsageBillingCommand_SubscriptionAppliesRateMultiplier(t *testing.T) {
+	t.Parallel()
+
+	groupID := int64(7)
+	subID := int64(42)
+
+	tests := []struct {
+		name           string
+		totalCost      float64
+		actualCost     float64
+		isSubscription bool
+		wantSub        float64
+		wantBalance    float64
+	}{
+		{
+			name:           "subscription with 2x multiplier consumes 2x quota",
+			totalCost:      1.0,
+			actualCost:     2.0,
+			isSubscription: true,
+			wantSub:        2.0,
+			wantBalance:    0,
+		},
+		{
+			name:           "subscription with 0.5x multiplier consumes 0.5x quota",
+			totalCost:      1.0,
+			actualCost:     0.5,
+			isSubscription: true,
+			wantSub:        0.5,
+			wantBalance:    0,
+		},
+		{
+			name:           "free subscription (multiplier 0) consumes no quota",
+			totalCost:      1.0,
+			actualCost:     0,
+			isSubscription: true,
+			wantSub:        0,
+			wantBalance:    0,
+		},
+		{
+			name:           "balance billing keeps using ActualCost (regression)",
+			totalCost:      1.0,
+			actualCost:     2.0,
+			isSubscription: false,
+			wantSub:        0,
+			wantBalance:    2.0,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			p := &postUsageBillingParams{
+				Cost:               &CostBreakdown{TotalCost: tt.totalCost, ActualCost: tt.actualCost},
+				User:               &User{ID: 1},
+				APIKey:             &APIKey{ID: 2, GroupID: &groupID},
+				Account:            &Account{ID: 3},
+				Subscription:       &UserSubscription{ID: subID},
+				IsSubscriptionBill: tt.isSubscription,
+			}
+
+			cmd := buildUsageBillingCommand("req-1", nil, p)
+			if cmd == nil {
+				t.Fatal("buildUsageBillingCommand returned nil")
+			}
+			if cmd.SubscriptionCost != tt.wantSub {
+				t.Errorf("SubscriptionCost = %v, want %v", cmd.SubscriptionCost, tt.wantSub)
+			}
+			if cmd.BalanceCost != tt.wantBalance {
+				t.Errorf("BalanceCost = %v, want %v", cmd.BalanceCost, tt.wantBalance)
+			}
+		})
+	}
+}