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/billing_service.go

@@ -221,6 +221,18 @@ func (s *BillingService) initFallbackPricing() {
 		LongContextInputMultiplier:     openAIGPT54LongContextInputMultiplier,
 		LongContextOutputMultiplier:    openAIGPT54LongContextOutputMultiplier,
 	}
+	s.fallbackPrices["gpt-5.4-mini"] = &ModelPricing{
+		InputPricePerToken:             7.5e-7,
+		OutputPricePerToken:            4.5e-6,
+		CacheReadPricePerToken:         7.5e-8,
+		SupportsCacheBreakdown:         false,
+	}
+	s.fallbackPrices["gpt-5.4-nano"] = &ModelPricing{
+		InputPricePerToken:     2e-7,
+		OutputPricePerToken:    1.25e-6,
+		CacheReadPricePerToken: 2e-8,
+		SupportsCacheBreakdown: false,
+	}
 	// OpenAI GPT-5.2（本地兜底）
 	s.fallbackPrices["gpt-5.2"] = &ModelPricing{
 		InputPricePerToken:             1.75e-6,
@@ -294,6 +306,10 @@ func (s *BillingService) getFallbackPricing(model string) *ModelPricing {
 	if strings.Contains(modelLower, "gpt-5") || strings.Contains(modelLower, "codex") {
 		normalized := normalizeCodexModel(modelLower)
 		switch normalized {
+		case "gpt-5.4-mini":
+			return s.fallbackPrices["gpt-5.4-mini"]
+		case "gpt-5.4-nano":
+			return s.fallbackPrices["gpt-5.4-nano"]
 		case "gpt-5.4":
 			return s.fallbackPrices["gpt-5.4"]
 		case "gpt-5.2":