fix: 预览模式配音降级处理 - ASR 不可用时走整段配音模式

- render_adapter.py: 修复 import 路径 (services -> apps.worker.services)
- unified_render_service.py: 方式B 条件调整
  - ASR 可用时走字幕对齐模式（方式B）
  - ASR 不可用时降级为整段配音模式（方式A），使用 subtitle.text 作为配音内容
- 解决预览模式下配音完全不生效的问题

关联 Issue #1294

apps/worker/video_processing/render_adapter.py

@@ -465,7 +465,7 @@ class RenderAdapter:
         失败不阻断主流程，返回 None。
         """
         try:
-            from services.asr_service_factory import get_asr_service
+            from apps.worker.services.asr_service_factory import get_asr_service
 
             return get_asr_service()
         except Exception as e:

apps/worker/video_processing/unified_render_service.py

@@ -679,20 +679,40 @@ class UnifiedRenderService:
                     len(top_text),
                 )
             # 方式B：voice_id + 自动字幕 → 字幕对齐配音（预设配音模式）
-            elif top_voice_id and subtitle_cfg.get("auto_generated", False) and self.asr_service is not None:
-                tts_cfg = {
-                    "enabled": True,
-                    "voice_id": top_voice_id,
-                    "text": "",
-                    "align_mode": "subtitle",
-                    "overlap_mode": "replace",
-                }
-                use_subtitle_align = True
-                logger.info(
-                    "[unified-render] 检测到预设配音+自动字幕，使用字幕对齐模式: plan_id=%s voice_id=%s",
-                    self.plan.id,
-                    top_voice_id,
-                )
+            # ASR 可用时走字幕对齐模式，不可用时降级为整段配音
+            elif top_voice_id and subtitle_cfg.get("auto_generated", False):
+                if self.asr_service is not None:
+                    # 字幕对齐模式
+                    tts_cfg = {
+                        "enabled": True,
+                        "voice_id": top_voice_id,
+                        "text": "",
+                        "align_mode": "subtitle",
+                        "overlap_mode": "replace",
+                    }
+                    use_subtitle_align = True
+                    logger.info(
+                        "[unified-render] 检测到预设配音+自动字幕，使用字幕对齐模式: plan_id=%s voice_id=%s",
+                        self.plan.id,
+                        top_voice_id,
+                    )
+                else:
+                    # 降级：整段配音（预览模式 ASR 不可用时）
+                    # 拼接字幕文本作为配音内容
+                    subtitle_text_content = subtitle_cfg.get("text", "") or ""
+                    tts_cfg = {
+                        "enabled": True,
+                        "voice_id": top_voice_id,
+                        "text": subtitle_text_content,
+                        "align_mode": "full",
+                        "overlap_mode": "replace",
+                    }
+                    logger.info(
+                        "[unified-render] ASR 不可用，降级为整段配音模式: plan_id=%s voice_id=%s text_len=%d",
+                        self.plan.id,
+                        top_voice_id,
+                        len(subtitle_text_content),
+                    )
 
         # 兼容前端顶层字段：voice_id / custom_text / voice_clone_profile_id
         # 前端一键生成页面传 config.voice_id + config.custom_text，