feat: 转场特效引擎 — TransitionEngine + 14种转场预设 + 时长边界校验 + 降级策略

核心能力：
- TransitionEngine 抽象层：TransitionType枚举 + TransitionConfig配置 + 引擎类
- 14种转场预设：fade/dissolve/zoom/circlecrop/rectcrop + slide*4 + wipe*4
- 转场时长边界校验：0.3s~2s范围，超出自动钳制
- 降级策略：不支持的转场自动fallback到硬切(cut)，不阻断渲染
- 别名支持：slide_left/slide-left/crossfade/wipe/zoomin等多种写法

系统集成：
- UnifiedRenderService 接入 TransitionEngine，替换直接xfade调用
- EditPlanClip 领域模型新增 transition_duration 字段
- SQLAlchemy model + repository 同步新增列
- API层 Schema + Service 同步支持 transition_duration
- 向前兼容：transition_duration=0 表示使用全局默认值

测试：
- 54个新增单测：枚举/配置解析/边界校验/降级策略/滤镜链构建/集成验证
- 84个渲染测试 + 16个适配器测试全绿，无回归

apps/worker/video_processing/ffmpeg_utils.py

@@ -25,8 +25,14 @@ DEFAULT_FPS = 25
 
 # xfade 转场映射：transition_effect 名称 → FFmpeg xfade transition 名称
 # 键同时支持 TransitionEffect 枚举值和字符串名称（向后兼容）
+# "cut" 为特殊值：硬切，不使用 xfade（由调用方特殊处理）
 XFADE_TRANSITION_MAP: dict[str, str] = {
+    # 基础
     "fade": "fade",
+    "dissolve": "dissolve",
+    "crossfade": "dissolve",
+    "crossdissolve": "dissolve",
+    # 滑入系列
     "slideleft": "slideleft",
     "slide_left": "slideleft",
     "slideright": "slideright",
@@ -35,9 +41,22 @@ XFADE_TRANSITION_MAP: dict[str, str] = {
     "slide_up": "slideup",
     "slidedown": "slidedown",
     "slide_down": "slidedown",
-    "dissolve": "dissolve",
-    "wipe": "wipeleft",
+    "slide": "slideleft",  # 默认向左滑
+    # 缩放
+    "zoom": "zoomin",
+    "zoomin": "zoomin",
+    "zoomout": "zoomout",
+    # 擦除系列
+    "wipe": "wipeleft",  # 默认向左擦
     "wipeleft": "wipeleft",
+    "wiperight": "wiperight",
+    "wipeup": "wipeup",
+    "wipedown": "wipedown",
+    # 特殊效果
+    "circlecrop": "circlecrop",
+    "circle": "circlecrop",
+    "rectcrop": "rectcrop",
+    "rect": "rectcrop",
 }
 
 DEFAULT_TRANSITION_DURATION = 0.5