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/openai_ws_forwarder.go

@@ -2302,6 +2302,7 @@ func (s *OpenAIGatewayService) forwardOpenAIWSV2(
 		RequestID:       responseID,
 		Usage:           *usage,
 		Model:           originalModel,
+		ServiceTier:     extractOpenAIServiceTier(reqBody),
 		ReasoningEffort: extractOpenAIReasoningEffort(reqBody, originalModel),
 		Stream:          reqStream,
 		OpenAIWSMode:    true,
@@ -2913,6 +2914,7 @@ func (s *OpenAIGatewayService) ProxyResponsesWebSocketFromClient(
 					RequestID:       responseID,
 					Usage:           usage,
 					Model:           originalModel,
+					ServiceTier:     extractOpenAIServiceTierFromBody(payload),
 					ReasoningEffort: extractOpenAIReasoningEffortFromBody(payload, originalModel),
 					Stream:          reqStream,
 					OpenAIWSMode:    true,