feat(material): remove video cover extraction and simplify upload API

- Remove extractVideoCoverOptional function and related video cover processing
- Update MaterialService.uploadFile method signature to remove coverBase64 parameter
- Simplify uploadAndIdentifyVideo function by removing cover generation logic
- Remove loading indicator from VideoSelector component during video preview
- Add presignGetUrlWithProcess method to FileClient interface for processed file URLs
- Add logging support to S3FileClient implementation

frontend/app/web-gold/src/composables/useUpload.js

@@ -47,7 +47,6 @@ function getVideoDuration(file) {
  * @typedef {Object} UploadOptions
  * @property {string} fileCategory - 文件分类（video/voice/audio/image/digital_human）
  * @property {number|null} groupId - 分组编号（可选，仅素材库模块使用）
- * @property {string|null} coverBase64 - 封面base64（可选）
  * @property {number|null} duration - 视频时长（可选，视频文件自动获取）
  * @property {Function} onProgress - 进度回调（可选）
  * @property {Function} onStart - 开始回调（可选）
@@ -132,7 +131,6 @@ export function useUpload() {
     const {
       fileCategory,
       groupId = null,
-      coverBase64 = null,
       duration: inputDuration,
       onProgress,
       onStart,
@@ -193,7 +191,6 @@ export function useUpload() {
         fileSize: file.size,
         fileType: file.type,
         groupId,
-        coverBase64,
         duration
       })