fix(media): 对齐 EasyAI 媒体响应与转存策略

统一媒体异步提交和轮询响应，扩展 /ai/result 到当前用户的 Gateway 媒体任务，并保持既有 Gateway 字段兼容。\n\n启用转存但无可用渠道时回退到 24 小时本地静态资源；关闭转存时保留图片、音频等上游 Base64 字段。同步更新文件上传兼容结构、OpenAPI、管理端说明和回归测试。\n\n验证：cd apps/api && env -u AI_GATEWAY_TEST_DATABASE_URL go test ./... -count=1；gofmt -l 无输出；pnpm nx run web:typecheck。

apps/api/internal/httpapi/task_idempotency_test.go

@@ -1,8 +1,12 @@
 package httpapi
 
 import (
+	"encoding/json"
+	"net/http"
 	"net/http/httptest"
 	"testing"
+
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
 )
 
 func TestOptionalTaskIdempotencyKeyRejectsMultipleValues(t *testing.T) {
@@ -29,3 +33,29 @@ func TestTaskIdempotencyRequestHashIsCanonical(t *testing.T) {
 		t.Fatal("async response semantics must be part of the request hash")
 	}
 }
+
+func TestWriteIdempotentAsyncTaskReplayKeepsEasyAICompatibilityFields(t *testing.T) {
+	t.Parallel()
+	task := store.GatewayTask{
+		ID:        "async-replay-1",
+		Kind:      "images.generations",
+		Status:    "queued",
+		AsyncMode: true,
+	}
+	recorder := httptest.NewRecorder()
+
+	writeIdempotentTaskReplay(recorder, task, true)
+
+	if recorder.Code != http.StatusAccepted ||
+		recorder.Header().Get("Idempotent-Replayed") != "true" ||
+		recorder.Header().Get("X-Gateway-Task-Id") != task.ID {
+		t.Fatalf("unexpected replay status/headers: code=%d headers=%v", recorder.Code, recorder.Header())
+	}
+	var got map[string]any
+	if err := json.NewDecoder(recorder.Body).Decode(&got); err != nil {
+		t.Fatal(err)
+	}
+	if got["status"] != "submitted" || got["task_id"] != task.ID || got["taskId"] != task.ID {
+		t.Fatalf("unexpected replay body: %+v", got)
+	}
+}