fix(billing): apply 2x service-tier multiplier for fast/priority fallback

Models without an explicit priority price table (e.g. gpt-5.4-mini, gpt-5.4-nano)
were billed at base rate when requested with service_tier=fast/priority, while
OpenAI's published priority pricing for those SKUs is exactly 2x base. Align
with sub2api: serviceTierCostMultiplier now returns 2.0 for priority/fast, and
models that *do* declare InputPricePerMTokenPriority continue to use their
explicit priority price table unchanged.

Fixes #158.

Author: james-6-23

database/billing.go

@@ -337,6 +337,8 @@ func usePriorityPricing(serviceTier string, pricing *ModelPricing) bool {
 
 func serviceTierCostMultiplier(serviceTier string) float64 {
 	switch normalizeServiceTier(serviceTier) {
+	case "priority", "fast":
+		return 2.0
 	case "flex":
 		return 0.5
 	default:

database/billing_test.go

@@ -109,24 +109,6 @@ func TestCalculateCostHandlesCachedTokensAndServiceTier(t *testing.T) {
 			cachedTokens: 200,
 			want:         0.0191,
 		},
-		{
-			name:         "does not invent priority multiplier when priority price is unknown",
-			model:        "gpt-4o",
-			serviceTier:  "priority",
-			inputTokens:  1000,
-			outputTokens: 500,
-			cachedTokens: 200,
-			want:         0.0075,
-		},
-		{
-			name:         "fast tier falls back to standard pricing when priority price is unknown",
-			model:        "gpt-4o",
-			serviceTier:  "fast",
-			inputTokens:  1000,
-			outputTokens: 500,
-			cachedTokens: 200,
-			want:         0.0075,
-		},
 		{
 			name:         "applies flex multiplier",
 			model:        "gpt-5.4",
@@ -161,6 +143,19 @@ func TestCalculateCostBreakdownExposesDisplayFields(t *testing.T) {
 	assertFloatEqual(t, got.ServiceTierCostMultiplier, 0.5)
 }
 
+func TestFastTierFallbackDoublesCostForModelsWithoutPriorityPricing(t *testing.T) {
+	for _, model := range []string{"gpt-5.4-mini", "gpt-5.4-nano"} {
+		base := calculateCostBreakdown(1000, 500, 200, model, "")
+		fast := calculateCostBreakdown(1000, 500, 200, model, "fast")
+		priority := calculateCostBreakdown(1000, 500, 200, model, "priority")
+
+		assertFloatEqual(t, fast.TotalCost, base.TotalCost*2)
+		assertFloatEqual(t, priority.TotalCost, base.TotalCost*2)
+		assertFloatEqual(t, fast.ServiceTierCostMultiplier, 2.0)
+		assertFloatEqual(t, priority.ServiceTierCostMultiplier, 2.0)
+	}
+}
+
 func TestGPT55PricingDoesNotMatchGPT54(t *testing.T) {
 	gpt54 := GetModelPricing("gpt-5.4")
 	gpt55 := GetModelPricing("gpt-5.5")