feat: 统一预览与确认生成渲染路径 — 预览=正式品质+确认复用

## 渲染管线统一
- UnifiedRenderService: 移除 is_preview 参数，统一 CRF 23 + medium preset
- RenderAdapter: 移除 is_preview 参数，统一执行校验和缩略图生成
- Worker generation.py: 移除 480p+1M 码率覆盖逻辑
- generation_preview.py: 删除 PREVIEW_RESOLUTION 常量和 _calc_preview_resolution()

## 确认生成复用预览产物
- Domain: 新增 GenerationTask.mark_confirmed() 方法
- confirm_generation: 预览已完成时直接复用产物（秒出），无需重新渲染
- Editor generate: 检查 plan 关联的预览任务，plan未修改时复用产物
- Schema: CreatePreviewGenerationTaskRequest 新增 source_edit_plan_id 字段

## 测试更新
- test_1280_preview_speedup: 重写为验证统一品质参数
- test_confirm_generation: 重写为验证复用逻辑

## 交付标准
- 预览渲染质量与确认生成一致（1080p, CRF 23, medium）
- 预览后确认生成直接复用产物（秒出）
- Worker 端无任何 is_preview 低质量渲染代码残留

apps/worker/video_processing/unified_render_service.py

@@ -151,7 +151,6 @@ class UnifiedRenderService:
         asr_service: Any = None,  # ASRService 实例，用于自动生成字幕
         bgm_path: str | None = None,  # BGM 本地文件路径
         voiceover_audio_path: str | None = None,  # 配音素材库音频本地路径
-        is_preview: bool = False,  # 预览模式：ultrafast 编码 + 跳过非必要步骤
     ):
         self.plan = plan
         self.clips = clips
@@ -164,7 +163,6 @@ class UnifiedRenderService:
         self.asr_service = asr_service
         self.bgm_path = bgm_path
         self.voiceover_audio_path = voiceover_audio_path
-        self.is_preview = is_preview
         self._transition_engine = TransitionEngine(default_duration=transition_duration)
         self._speed_engine = SpeedEngine()
         self._asr_timeline_cache: Any = None  # ASR 字幕结果缓存，避免重复调用
@@ -1250,9 +1248,9 @@ class UnifiedRenderService:
             "-c:v",
             "libx264",
             "-crf",
-            "28" if self.is_preview else "23",
+            "23",
             "-preset",
-            "ultrafast" if self.is_preview else "medium",
+            "medium",
             "-pix_fmt",
             "yuv420p",
             "-movflags",
@@ -1776,9 +1774,9 @@ class UnifiedRenderService:
             "-c:v",
             "libx264",
             "-crf",
-            "28" if self.is_preview else "23",
+            "23",
             "-preset",
-            "ultrafast" if self.is_preview else "medium",
+            "medium",
             "-pix_fmt",
             "yuv420p",
             "-movflags",
@@ -1787,11 +1785,10 @@ class UnifiedRenderService:
         ]
 
         logger.info(
-            "执行渲染: plan_id=%s inputs=%d output=%s preview=%s",
+            "执行渲染: plan_id=%s inputs=%d output=%s",
             self.plan.id,
             input_args.count("-i"),
             output_path,
-            self.is_preview,
         )
         try:
             run_ffmpeg(command)