perf(#1280): 预览视频生成加速

优化项：
1. FFmpeg 编码加速：预览模式使用 -preset ultrafast + crf 28（原 medium + crf 23）
2. 并行素材下载：使用 ThreadPoolExecutor 并发下载（最多6线程），保持原始顺序
3. 跳过 ASR 初始化：预览模式不需要自动字幕
4. 跳过输出校验：预览模式跳过 validate_video_output
5. 跳过缩略图生成：预览模式不生成封面缩略图

改动文件：
- apps/worker/video_processing/unified_render_service.py：is_preview 参数
- apps/worker/video_processing/render_adapter.py：is_preview 透传 + 跳过非必要步骤
- apps/worker/worker_app/tasks/generation.py：并行下载 + is_preview 透传
- tests/unit/test_1280_preview_speedup.py：13 个新测试

预期效果：30s 视频预览生成从 ~2-3min 降至 ~30-60s

apps/worker/video_processing/render_adapter.py

@@ -485,6 +485,7 @@ class RenderAdapter:
         rendered_clip_ids: list[str] | None = None,
         failed_clip_ids: list[str] | None = None,
         voiceover_audio_path: str | None = None,
+        is_preview: bool = False,
     ) -> RenderAdapterResult:
         """执行统一渲染核心流程（BGM + ASR + 渲染 + 缩略图 + 上传）。
 
@@ -503,8 +504,8 @@ class RenderAdapter:
 
         self._report_progress(progress_cb, 40.0, "执行视频渲染")
 
-        # 2. 初始化 ASR
-        asr_service = self._get_asr_service()
+        # 2. 初始化 ASR（预览模式跳过，节省启动开销）
+        asr_service = None if is_preview else self._get_asr_service()
 
         # 3. 读取输出分辨率
         plan_config = plan.config or {}
@@ -529,25 +530,27 @@ class RenderAdapter:
             bgm_path=bgm_path,
             asr_service=asr_service,
             voiceover_audio_path=voiceover_audio_path,
+            is_preview=is_preview,
         )
         result = render_svc.render()
 
-        # 4.5 渲染后校验输出完整性
-
-        validation = validate_video_output(result.output_path)
-        if not validation.valid:
-            logger.error(
-                "[render-adapter] 渲染输出校验失败: plan_id=%s job_id=%s error=%s",
-                plan_id,
-                job_id,
-                validation.error_message,
-            )
-            return RenderAdapterResult(
-                success=False,
-                error_message=f"渲染输出校验失败: {validation.error_message}",
-                error_detail=validation.error_message,
-            )
-
+        # 4.5 渲染后校验输出完整性（预览模式跳过，节省耗时）
+        if is_preview:
+            logger.info("[render-adapter] 预览模式：跳过输出校验")
+        else:
+            validation = validate_video_output(result.output_path)
+            if not validation.valid:
+                logger.error(
+                    "[render-adapter] 渲染输出校验失败: plan_id=%s job_id=%s error=%s",
+                    plan_id,
+                    job_id,
+                    validation.error_message,
+                )
+                return RenderAdapterResult(
+                    success=False,
+                    error_message=f"渲染输出校验失败: {validation.error_message}",
+                    error_detail=validation.error_message,
+                )
         self._report_progress(progress_cb, 80.0, "上传渲染结果")
 
         # 5. 上传结果
@@ -556,19 +559,20 @@ class RenderAdapter:
 
         self._report_progress(progress_cb, 90.0, "生成封面缩略图")
 
-        # 6. 生成缩略图
+        # 6. 生成缩略图（预览模式跳过，节省耗时）
         thumbnail_url = ""
-        try:
-            from video_processing.thumbnail_generator import generate_and_upload_thumbnail
+        if not is_preview:
+            try:
+                from video_processing.thumbnail_generator import generate_and_upload_thumbnail
 
-            thumb_storage_key = f"rendered/{plan_id}/thumbnail.jpg"
-            thumbnail_url = generate_and_upload_thumbnail(str(result.output_path), thumb_storage_key)
-        except Exception as thumb_err:
-            logger.warning(
-                "[render-adapter] 缩略图生成失败（不影响主流程）: plan_id=%s error=%s",
-                plan_id,
-                thumb_err,
-            )
+                thumb_storage_key = f"rendered/{plan_id}/thumbnail.jpg"
+                thumbnail_url = generate_and_upload_thumbnail(str(result.output_path), thumb_storage_key)
+            except Exception as thumb_err:
+                logger.warning(
+                    "[render-adapter] 缩略图生成失败（不影响主流程）: plan_id=%s error=%s",
+                    plan_id,
+                    thumb_err,
+                )
 
         self._report_progress(progress_cb, 100.0, "渲染完成")
 
@@ -614,6 +618,7 @@ class RenderAdapter:
         work_dir: Path | None = None,
         progress_cb: ProgressCallback | None = None,
         voiceover_audio_path: str | None = None,
+        is_preview: bool = False,
     ) -> RenderAdapterResult:
         """使用内存中的 plan/clips/asset_path_map 直接渲染。
 
@@ -672,6 +677,7 @@ class RenderAdapter:
                 job_id=job_id,
                 progress_cb=progress_cb,
                 voiceover_audio_path=voiceover_audio_path,
+                is_preview=is_preview,
             )
 
         except subprocess.CalledProcessError as exc: