feat: 视频调速引擎（快进/慢放）

- SpeedEngine 抽象层：SpeedConfig + SpeedEngine，支持 0.25x~4x 变速
- 视频调速：基于 FFmpeg setpts，在 clip 预处理环节插入
- 音频调速：基于 atempo，超范围自动多级串联（如 4x=2.0*2.0）
- 音画同步：视频音频同时调速，音调自动修正
- 分段调速：每个 clip 独立 playback_speed 字段
- 整段调速：所有 clip 设相同 speed 即可实现
- 降级策略：速度超出范围自动钳制，不阻断渲染
- 领域模型：EditPlanClip 新增 playback_speed 字段
- 数据库：EditPlanClipModel 新增 playback_speed 列
- API：create_clip / update_clip 支持 playback_speed 参数
- 45 个新增单测全绿 + 100 个现有测试全绿，无回归

packages/adapters/sqlalchemy_impl/edit_plan_clip_repository.py

@@ -55,6 +55,7 @@ class SQLAlchemyEditPlanClipRepository:
             duration=clip.duration,
             transition_effect=clip.transition_effect,
             transition_duration=clip.transition_duration,
+            playback_speed=clip.playback_speed,
             status=clip.status,
             config=clip.config,
         )
@@ -78,6 +79,7 @@ class SQLAlchemyEditPlanClipRepository:
         model.duration = clip.duration
         model.transition_effect = clip.transition_effect
         model.transition_duration = clip.transition_duration
+        model.playback_speed = clip.playback_speed
         model.status = clip.status
         model.config = clip.config
         model.updated_at = clip.updated_at
@@ -123,6 +125,7 @@ class SQLAlchemyEditPlanClipRepository:
             duration=model.duration or 0.0,
             transition_effect=model.transition_effect or "cut",
             transition_duration=getattr(model, "transition_duration", 0.0) or 0.0,
+            playback_speed=model.playback_speed or 1.0,
             status=EditPlanClipStatus(model.status) if model.status else EditPlanClipStatus.PENDING,
             config=model.config or {},
             created_at=model.created_at,

packages/adapters/sqlalchemy_impl/models.py

@@ -197,6 +197,7 @@ class EditPlanClipModel(Base):
     duration = Column(Float, nullable=False, default=0.0)
     transition_effect = Column(String(20), nullable=False, default="cut")
     transition_duration = Column(Float, nullable=False, default=0.0)
+    playback_speed = Column(Float, nullable=False, default=1.0)
     status = Column(String(20), nullable=False, default="pending", index=True)
     config = Column(JSON, nullable=False, default=dict)
     created_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))