feat(Sora): 直连生成并移除sora2api依赖

实现直连 Sora 客户端、媒体落地与清理策略\n更新网关与前端配置以支持 Sora 平台\n补齐单元测试与契约测试，新增 curl 测试脚本\n\n测试: go test ./... -tags=unit

backend/internal/service/sora_media_storage_test.go

@@ -0,0 +1,69 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSoraMediaStorage_StoreFromURLs(t *testing.T) {
+	tmpDir := t.TempDir()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "image/png")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("data"))
+	}))
+	defer server.Close()
+
+	cfg := &config.Config{
+		Sora: config.SoraConfig{
+			Storage: config.SoraStorageConfig{
+				Type:                   "local",
+				LocalPath:              tmpDir,
+				MaxConcurrentDownloads: 1,
+			},
+		},
+	}
+
+	storage := NewSoraMediaStorage(cfg)
+	urls, err := storage.StoreFromURLs(context.Background(), "image", []string{server.URL + "/img.png"})
+	require.NoError(t, err)
+	require.Len(t, urls, 1)
+	require.True(t, strings.HasPrefix(urls[0], "/image/"))
+	require.True(t, strings.HasSuffix(urls[0], ".png"))
+
+	localPath := filepath.Join(tmpDir, filepath.FromSlash(strings.TrimPrefix(urls[0], "/")))
+	require.FileExists(t, localPath)
+}
+
+func TestSoraMediaStorage_FallbackToUpstream(t *testing.T) {
+	tmpDir := t.TempDir()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer server.Close()
+
+	cfg := &config.Config{
+		Sora: config.SoraConfig{
+			Storage: config.SoraStorageConfig{
+				Type:               "local",
+				LocalPath:          tmpDir,
+				FallbackToUpstream: true,
+			},
+		},
+	}
+
+	storage := NewSoraMediaStorage(cfg)
+	url := server.URL + "/broken.png"
+	urls, err := storage.StoreFromURLs(context.Background(), "image", []string{url})
+	require.NoError(t, err)
+	require.Equal(t, []string{url}, urls)
+}