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/openai_codex_transform.go

@@ -7,6 +7,8 @@ import (
 
 var codexModelMap = map[string]string{
 	"gpt-5.4":                    "gpt-5.4",
+	"gpt-5.4-mini":               "gpt-5.4-mini",
+	"gpt-5.4-nano":               "gpt-5.4-nano",
 	"gpt-5.4-none":               "gpt-5.4",
 	"gpt-5.4-low":                "gpt-5.4",
 	"gpt-5.4-medium":             "gpt-5.4",
@@ -225,6 +227,12 @@ func normalizeCodexModel(model string) string {
 
 	normalized := strings.ToLower(modelID)
 
+	if strings.Contains(normalized, "gpt-5.4-mini") || strings.Contains(normalized, "gpt 5.4 mini") {
+		return "gpt-5.4-mini"
+	}
+	if strings.Contains(normalized, "gpt-5.4-nano") || strings.Contains(normalized, "gpt 5.4 nano") {
+		return "gpt-5.4-nano"
+	}
 	if strings.Contains(normalized, "gpt-5.4") || strings.Contains(normalized, "gpt 5.4") {
 		return "gpt-5.4"
 	}