feat: 完善媒体管理与备份迁移流程

- 视频上传生成缩略图并提供占位封面和失败原因展示
- 支持 HLS 按需重新封装 MP4，并限制临时文件生命周期与访问权限
- 完善视频、文件 API 文档、搜索、批量删除和单次转码质量参数
- 修复备份恢复与跨后端迁移遗漏缩略图、HLS 清单和分片的问题
- 将备份任务、归档和下载权限按用户隔离
- 增加仅清理当前用户数据的高危存储清空流程和二次文字确认
- 修复 S3 中文文件名元数据及 CLI 数据库连接问题
- 补充备份恢复、用户隔离、媒体资源和转码相关测试

app/components/VideoCard.vue

@@ -16,11 +16,13 @@ const props = defineProps<{
 const emit = defineEmits<{
   'play': []
   'delete': []
+  'download': []
   'update:selected': [value: boolean]
 }>()
 
 const { formatFileSize } = useFileSize()
 const { t, locale } = useI18n()
+const transcodeFailureLabel = useTranscodeFailureLabel()
 
 /** 转码完成后按 HLS 播放列表展示 */
 const displayName = computed(() => {
@@ -37,34 +39,15 @@ const uploadedLabel = computed(() => {
     return props.item.uploadedAt
   }
 })
-
-const thumbFailed = ref(false)
-
-watch(() => props.item.thumbUrl, () => {
-  thumbFailed.value = false
-})
 </script>
 
 <template>
   <div class="flex flex-col overflow-hidden rounded-xl border border-default bg-elevated">
     <div class="relative aspect-video shrink-0 overflow-hidden bg-muted">
-      <img
-        v-if="item.thumbUrl && !thumbFailed"
+      <VideoThumbnail
         :src="thumbLink"
         :alt="displayName"
-        loading="lazy"
-        class="size-full object-cover object-center"
-        @error="thumbFailed = true"
-      >
-      <div
-        v-else
-        class="flex size-full items-center justify-center"
-      >
-        <UIcon
-          name="i-lucide-film"
-          class="size-10 text-dimmed"
-        />
-      </div>
+      />
 
       <button
         type="button"
@@ -108,8 +91,10 @@ watch(() => props.item.thumbUrl, () => {
           color="error"
           variant="solid"
           size="xs"
+          class="max-w-48 truncate"
+          :title="transcodeFailureLabel(item.transcodeError)"
         >
-          {{ t('library.transcodeFailed') }}
+          {{ transcodeFailureLabel(item.transcodeError) }}
         </UBadge>
       </div>
     </div>
@@ -162,6 +147,15 @@ watch(() => props.item.thumbUrl, () => {
         rel="noopener"
         :aria-label="t('files.download')"
       />
+      <UButton
+        v-else-if="item.hlsUrl"
+        icon="i-lucide-download"
+        variant="ghost"
+        color="neutral"
+        size="sm"
+        :aria-label="t('library.video.downloadMp4')"
+        @click="emit('download')"
+      />
       <UButton
         icon="i-lucide-trash-2"
         variant="ghost"