fix: extract and log Claude output_config.effort in usage records

Claude's output_config.effort parameter (low/medium/high/max) was not
being extracted from requests or logged in the reasoning_effort column
of usage logs. Only the OpenAI path populated this field.

Changes:
- Extract output_config.effort in ParseGatewayRequest
- Add ReasoningEffort field to ForwardResult
- Populate reasoning_effort in both RecordUsage and RecordUsageWithLongContext
- Guard against overwriting service-set effort values in handler
- Update stale comments that described reasoning_effort as OpenAI-only
- Add unit tests for extraction, normalization, and persistence

backend/internal/handler/gateway_handler.go

@@ -436,6 +436,10 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			clientIP := ip.GetClientIP(c)
 			requestPayloadHash := service.HashUsageRequestPayload(body)
 
+			if result.ReasoningEffort == nil {
+				result.ReasoningEffort = service.NormalizeClaudeOutputEffort(parsedReq.OutputEffort)
+			}
+
 			// 使用量记录通过有界 worker 池提交，避免请求热路径创建无界 goroutine。
 			h.submitUsageRecordTask(func(ctx context.Context) {
 				if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
@@ -740,6 +744,10 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			clientIP := ip.GetClientIP(c)
 			requestPayloadHash := service.HashUsageRequestPayload(body)
 
+			if result.ReasoningEffort == nil {
+				result.ReasoningEffort = service.NormalizeClaudeOutputEffort(parsedReq.OutputEffort)
+			}
+
 			// 使用量记录通过有界 worker 池提交，避免请求热路径创建无界 goroutine。
 			h.submitUsageRecordTask(func(ctx context.Context) {
 				if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{