fix(worker): 将 Gemini 同步请求交由异步队列执行

将非流式 Gemini generateContent 改为 River Worker 执行，API 通过批量状态查询等待完成，避免高并发同步请求耗尽 API 数据库连接池。\n\n生成图片在持久化时保存带哈希的资产引用，响应恢复时下载并校验后重建 Base64，数据库不保存媒体原文。请求体物化增加前置内存门禁，并扩展双 API/Worker PostgreSQL 压力测试。\n\n验证：go test ./... -count=1；go vet ./...；pnpm openapi；迁移安全检查；64 与 256 请求双角色异步 Gemini 压力测试。

apps/api/internal/httpapi/request_preparation.go

@@ -217,6 +217,38 @@ func (s *Server) acquireMediaRequestBodySlot(ctx context.Context) (func(), error
 	}
 }
 
+type mediaRequestBodyLeaseContextKey struct{}
+
+type mediaRequestBodyLease struct {
+	release func()
+}
+
+func (l *mediaRequestBodyLease) Release() {
+	if l != nil && l.release != nil {
+		l.release()
+	}
+}
+
+func (s *Server) withMediaRequestBodySlot(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		release, err := s.acquireMediaRequestBodySlot(r.Context())
+		if err != nil {
+			return
+		}
+		lease := &mediaRequestBodyLease{release: release}
+		defer lease.Release()
+		ctx := context.WithValue(r.Context(), mediaRequestBodyLeaseContextKey{}, lease)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}
+
+func (s *Server) requestMediaBodyRelease(ctx context.Context) (func(), error) {
+	if lease, ok := ctx.Value(mediaRequestBodyLeaseContextKey{}).(*mediaRequestBodyLease); ok && lease != nil {
+		return lease.Release, nil
+	}
+	return s.acquireMediaRequestBodySlot(ctx)
+}
+
 func requestAssetFromValue(key string, path []string, value any, siblings map[string]any) (decodedRequestAsset, bool, error) {
 	text, ok := value.(string)
 	if !ok {