feat(openai): 增加 gpt-5.4-mini/nano 模型支持与定价配置

- 接入 gpt-5.4-mini/nano 模型识别与规范化，补充默认模型列表
- 增加 gpt-5.4-mini/nano 输入/缓存命中/输出价格与计费兜底逻辑
- 同步前端模型白名单与 OpenCode 配置
- 补充 service tier(priority/flex) 计费回归测试

backend/internal/service/pricing_service_test.go

@@ -98,6 +98,36 @@ func TestGetModelPricing_Gpt54UsesStaticFallbackWhenRemoteMissing(t *testing.T)
 	require.InDelta(t, 1.5, got.LongContextOutputCostMultiplier, 1e-12)
 }
 
+func TestGetModelPricing_Gpt54MiniUsesDedicatedStaticFallbackWhenRemoteMissing(t *testing.T) {
+	svc := &PricingService{
+		pricingData: map[string]*LiteLLMModelPricing{
+			"gpt-5.1-codex": {InputCostPerToken: 1.25e-6},
+		},
+	}
+
+	got := svc.GetModelPricing("gpt-5.4-mini")
+	require.NotNil(t, got)
+	require.InDelta(t, 7.5e-7, got.InputCostPerToken, 1e-12)
+	require.InDelta(t, 4.5e-6, got.OutputCostPerToken, 1e-12)
+	require.InDelta(t, 7.5e-8, got.CacheReadInputTokenCost, 1e-12)
+	require.Zero(t, got.LongContextInputTokenThreshold)
+}
+
+func TestGetModelPricing_Gpt54NanoUsesDedicatedStaticFallbackWhenRemoteMissing(t *testing.T) {
+	svc := &PricingService{
+		pricingData: map[string]*LiteLLMModelPricing{
+			"gpt-5.1-codex": {InputCostPerToken: 1.25e-6},
+		},
+	}
+
+	got := svc.GetModelPricing("gpt-5.4-nano")
+	require.NotNil(t, got)
+	require.InDelta(t, 2e-7, got.InputCostPerToken, 1e-12)
+	require.InDelta(t, 1.25e-6, got.OutputCostPerToken, 1e-12)
+	require.InDelta(t, 2e-8, got.CacheReadInputTokenCost, 1e-12)
+	require.Zero(t, got.LongContextInputTokenThreshold)
+}
+
 func TestParsePricingData_PreservesPriorityAndServiceTierFields(t *testing.T) {
 	raw := map[string]any{
 		"gpt-5.4": map[string]any{