fix(billing): 修复 OpenAI fast 档位计费并补齐展示

- 打通 service_tier 在 OpenAI HTTP、WS、passthrough 与 usage 记录中的传递
- 修正 priority/flex 计费逻辑，并将 fast 归一化为 priority
- 在用户端和管理端补齐服务档位与计费明细展示
- 补齐前后端测试，并修复 WS 限流信号重复持久化导致的全量回归失败

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/usage_log.go

@@ -98,6 +98,8 @@ type UsageLog struct {
 	AccountID int64
 	RequestID string
 	Model     string
+	// ServiceTier records the OpenAI service tier used for billing, e.g. "priority" / "flex".
+	ServiceTier *string
 	// ReasoningEffort is the request's reasoning effort level (OpenAI Responses API),
 	// e.g. "low" / "medium" / "high" / "xhigh". Nil means not provided / not applicable.
 	ReasoningEffort *string