perf(queue): 限制大媒体任务内存并消除准入惊群

将同步与异步非文本任务的准入唤醒改为按任务和全局队首推进，批量续租等待者，避免大量请求同时争抢 advisory lock 和数据库连接。

对 Base64 请求解析、素材解码、上游媒体执行和结果物化增加分层并发限制，复用请求素材并释放重复 Gemini wire 数据；生产 API 默认入口解析并发 16，媒体物化并发 8。

新增 1000 个同步 Gemini 图像编辑请求的模拟上游压力验收，10 MiB 输入和输出下全部成功，Heap 峰值增长约 2.58 GiB，并验证共享上传哈希、单次 attempt 与本地零落盘。

验证：go test ./... -count=1；go vet ./...；gofmt -l 无输出；kubectl kustomize deploy/kubernetes/production；10 MiB Gemini Base64 千任务压力测试通过。

apps/api/internal/httpapi/gemini_compat.go

@@ -117,6 +117,15 @@ func (s *Server) geminiGenerateContent(w http.ResponseWriter, r *http.Request) {
 		writeGeminiTaskError(http.StatusUnauthorized, "unauthorized", nil, "unauthorized")
 		return
 	}
+	releaseRequestBody, err := s.acquireMediaRequestBodySlot(r.Context())
+	if err != nil {
+		return
+	}
+	defer func() {
+		if releaseRequestBody != nil {
+			releaseRequestBody()
+		}
+	}()
 	var native map[string]any
 	if err := json.NewDecoder(r.Body).Decode(&native); err != nil {
 		writeGeminiTaskError(http.StatusBadRequest, "invalid json body", nil, "invalid_json_body")
@@ -150,6 +159,13 @@ func (s *Server) geminiGenerateContent(w http.ResponseWriter, r *http.Request) {
 		writeGeminiTaskError(status, err.Error(), nil, clients.ErrorCode(err))
 		return
 	}
+	// The queued task only needs the materialized assetRef. Release the native
+	// Base64 request body before it waits for distributed admission or an
+	// upstream response.
+	native = nil
+	mapping.Body = nil
+	releaseRequestBody()
+	releaseRequestBody = nil
 	createInput := store.CreateTaskInput{
 		Kind:            mapping.Kind,
 		Model:           mapping.Model,