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_cleanup_service_test.go

@@ -0,0 +1,46 @@
+//go:build unit
+
+package service
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSoraMediaCleanupService_RunCleanup(t *testing.T) {
+	tmpDir := t.TempDir()
+	cfg := &config.Config{
+		Sora: config.SoraConfig{
+			Storage: config.SoraStorageConfig{
+				Type:      "local",
+				LocalPath: tmpDir,
+				Cleanup: config.SoraStorageCleanupConfig{
+					Enabled:       true,
+					RetentionDays: 1,
+				},
+			},
+		},
+	}
+
+	storage := NewSoraMediaStorage(cfg)
+	require.NoError(t, storage.EnsureLocalDirs())
+
+	oldImage := filepath.Join(storage.ImageRoot(), "old.png")
+	newVideo := filepath.Join(storage.VideoRoot(), "new.mp4")
+	require.NoError(t, os.WriteFile(oldImage, []byte("old"), 0o644))
+	require.NoError(t, os.WriteFile(newVideo, []byte("new"), 0o644))
+
+	oldTime := time.Now().Add(-48 * time.Hour)
+	require.NoError(t, os.Chtimes(oldImage, oldTime, oldTime))
+
+	cleanup := NewSoraMediaCleanupService(storage, cfg)
+	cleanup.runCleanup()
+
+	require.NoFileExists(t, oldImage)
+	require.FileExists(t, newVideo)
+}