fix(service): 修复 Sora 媒体落地路径穿越风险

- 新增安全路径拼接校验，确保目标文件仍在下载目录内
- 清理失败下载文件时复用安全校验，避免不安全删除路径
- 增加扩展名白名单归一化与相关单元测试

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/sora_media_storage_test.go

@@ -91,3 +91,21 @@ func TestSoraMediaStorage_MaxDownloadBytes(t *testing.T) {
 	_, err := storage.StoreFromURLs(context.Background(), "image", []string{server.URL + "/img.png"})
 	require.Error(t, err)
 }
+
+func TestJoinPathWithinDir(t *testing.T) {
+	baseDir := t.TempDir()
+
+	path1, err := joinPathWithinDir(baseDir, "ok.png")
+	require.NoError(t, err)
+	require.Equal(t, filepath.Join(baseDir, "ok.png"), path1)
+
+	_, err = joinPathWithinDir(baseDir, "../escape.png")
+	require.Error(t, err)
+}
+
+func TestNormalizeSoraFileExt(t *testing.T) {
+	require.Equal(t, ".png", normalizeSoraFileExt(".PNG"))
+	require.Equal(t, ".mp4", normalizeSoraFileExt(".mp4"))
+	require.Equal(t, "", normalizeSoraFileExt("../../etc/passwd"))
+	require.Equal(t, "", normalizeSoraFileExt(".php"))
+}