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/views/material/components/SceneSelectorModal.vue

@@ -69,7 +69,7 @@
           @dblclick="handleQuickConfirm(file)"
         >
           <div class="card-cover">
-            <img v-if="file.coverBase64" :src="file.coverBase64" />
+            <img v-if="file.previewUrl" :src="file.previewUrl" />
             <div v-else class="cover-placeholder">
               <VideoCameraOutlined />
             </div>