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/request_preparation_test.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/config"
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
 )
 
 func TestRequestAssetFromValueDetectsDataURLAndRawBase64(t *testing.T) {
@@ -66,6 +67,33 @@ func TestRequestAssetFromValueDetectsGeminiInlineData(t *testing.T) {
 	}
 }
 
+func TestRequestAssetStillUsableRequiresExistingLocalFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "asset.png")
+	if err := os.WriteFile(path, []byte("image"), 0o600); err != nil {
+		t.Fatalf("write local request asset: %v", err)
+	}
+	asset := store.RequestAsset{
+		URL:             "http://127.0.0.1/static/uploaded/asset.png",
+		StorageProvider: "local_static",
+		LocalPath:       path,
+		ByteSize:        5,
+	}
+	if !requestAssetStillUsable(asset, time.Now()) {
+		t.Fatal("existing local request asset was rejected")
+	}
+	asset.ByteSize = 6
+	if requestAssetStillUsable(asset, time.Now()) {
+		t.Fatal("truncated local request asset was treated as reusable")
+	}
+	asset.ByteSize = 5
+	if err := os.Remove(path); err != nil {
+		t.Fatalf("remove local request asset: %v", err)
+	}
+	if requestAssetStillUsable(asset, time.Now()) {
+		t.Fatal("missing local request asset was treated as reusable")
+	}
+}
+
 func TestCanonicalConversationMessageHashUsesTextAndAssetRefs(t *testing.T) {
 	message := map[string]any{
 		"role": "user",