fix(billing): bill composite alias requests by the concrete forwarded model

Composite public aliases (e.g. all/claude) reach the Anthropic/Gemini
billing core via OriginalModel/ChannelMappedModel source overrides.
Unknown aliases resolved to no pricing and silently recorded $0 cost,
while family-word aliases were mispriced by the fallback family match
(Opus traffic billed at the Sonnet fallback rate). The OpenAI path
already guards this via usageBillingModelCandidates; the shared
recordUsageCore had neither the guard nor a fallback.

- composite groups: unless the admin explicitly configured channel
  pricing for the alias (OpenRouter-style custom pricing), bill by the
  concrete forwarded model
- general safety net: when the selected billing model has no resolvable
  pricing at all, fall back to the concrete forwarded model instead of
  silently recording $0
- grok media usage records now attribute OriginalModel to the client
  requested public alias, consistent with every other endpoint
  (billing unaffected: empty BillingModelSource never triggers source
  overrides)

Priced traffic and non-composite groups are unaffected.

backend/internal/handler/grok_media.go

@@ -472,8 +472,11 @@ func recordGrokMediaUsage(
 	inboundEndpoint := GetInboundEndpoint(c)
 	upstreamEndpoint := GetUpstreamEndpoint(c, account.Platform)
 	quotaPlatform := service.QuotaPlatform(c.Request.Context(), apiKey)
+	// OriginalModel 记录客户端请求的模型：composite 分组下 body 已被改写为具体模型，
+	// 公开别名需从 context 取回，与其他端点的用量归因口径一致（计费不受影响：
+	// BillingModelSource 为空不会触发来源覆盖）。
 	channelUsageFields := service.ChannelUsageFields{
-		OriginalModel:      requestModel,
+		OriginalModel:      clientRequestedModel(c, requestModel),
 		ChannelMappedModel: requestModel,
 	}
 	h.submitOpenAIUsageRecordTask(c.Request.Context(), result, func(ctx context.Context) {