fix: 支持备份恢复与站点同步的大文件上传 版本更新到1.5.4

- 备份恢复的 multipart 分支改用 Web FormData，移除旧解析器约 128 MB 的限制
- 初始化恢复新增 application/octet-stream 流式写盘，前端改为直接上传文件流，避免整包进入内存
- 站点同步改用 Web FormData，并在读取请求体前拒绝超过 500 MB 的文件
- 新增 multipart 字段、重复字段和文件读取回归测试

验证：
- 2 GB 测试备份包流式恢复预览成功，服务内存保持低位
- 130 MB 站点同步上传成功
- 原有备份预览及图片、视频、文件上传正常
- npm test 234 项通过；npm run typecheck 与 eslint 通过

server/utils/multipart-form.test.ts

@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest'
+import { readFormData, type H3Event } from 'h3'
+import {
+  readFormDataFieldValues,
+  readFormDataFiles
+} from './multipart-form'
+
+async function parseFormData(form: FormData): Promise<FormData> {
+  const request = new Request('http://localhost/upload', {
+    method: 'POST',
+    body: form
+  })
+  return readFormData({ web: { request } } as unknown as H3Event)
+}
+
+describe('web FormData helpers', () => {
+  it('reads text fields and files from a multipart request', async () => {
+    const form = new FormData()
+    form.append('key', ' videos/2026/10/test.mov ')
+    form.append('tagNames', '["a","b"]')
+    form.append(
+      'file',
+      new File([new Uint8Array(1024)], 'test.mov', { type: 'video/quicktime' })
+    )
+
+    const parsed = await parseFormData(form)
+
+    expect(parsed.get('key')).toBe(' videos/2026/10/test.mov ')
+    expect(readFormDataFieldValues(parsed, 'tagNames')).toEqual(['a', 'b'])
+    expect(readFormDataFiles(parsed, ['file'])[0]).toMatchObject({
+      name: 'test.mov',
+      size: 1024,
+      type: 'video/quicktime'
+    })
+  })
+
+  it('keeps repeated field values and ignores empty files', async () => {
+    const form = new FormData()
+    form.append('tagNames', 'a,b')
+    form.append('tagNames', 'c')
+    form.append('files', new File([], 'empty.txt'))
+
+    const parsed = await parseFormData(form)
+
+    expect(readFormDataFieldValues(parsed, 'tagNames')).toEqual(['a,b', 'c'])
+    expect(readFormDataFiles(parsed, ['files'])).toEqual([])
+  })
+})