feat(video-pipeline): 添加可配置的转场策略引擎和TTS语速支持

- 引入转场策略系统（`getTransition`），支持 `fixed`、`director`、`rhythm` 三种模式
- 根据账号配置文件动态读取转场配置（`loadTransitions`）
- 图片和视频轨道分别调用转场策略，替代原有的固定“闪白”转场
- 支持 `byPosition`（hook/body/keypoint/closing）和 `byDirector` 两种高级选择策略
- 图片动画支持 `loop_animation` 与 `in_animation` 解析（“缩放”、“弹入”等组合）
- TTS 合成新增 `rate` 字段（源自账号配置 `ttsRate`），默认语速调整为 1.1
- 默认动画类型从 `kenburns-zoom` 改为 `缩放`，适配中文 CapCut

accounts/_template/account.json

@@ -21,6 +21,21 @@
       "highlightColor": "#FF6B35",
       "bold": true
     },
-    "defaultBGM": ""
+    "defaultBGM": "",
+    "transitions": {
+      "strategy": "rhythm",
+      "default": { "name": "闪白", "duration": 150000 },
+      "byPosition": {
+        "hook":    { "name": "闪白", "duration": 100000 },
+        "keypoint": { "name": "闪白", "duration": 120000 },
+        "body":    { "name": "溶解", "duration": 300000 },
+        "closing": { "name": "黑场", "duration": 200000 }
+      },
+      "byDirector": {
+        "tarantino": { "name": "闪白", "duration": 100000 },
+        "kitano":    { "name": "溶解", "duration": 400000 },
+        "fincher":   { "name": "黑场", "duration": 200000 }
+      }
+    }
   }
 }

accounts/军事账号/account.json

@@ -36,6 +36,21 @@
       "inAnimation": "淡入",
       "outAnimation": "淡出"
     },
-    "defaultBGM": ""
+    "defaultBGM": "",
+    "transitions": {
+      "strategy": "rhythm",
+      "default": { "name": "闪白", "duration": 150000 },
+      "byPosition": {
+        "hook":     { "name": "闪白", "duration": 100000 },
+        "keypoint": { "name": "闪白", "duration": 120000 },
+        "body":     { "name": "溶解", "duration": 300000 },
+        "closing":  { "name": "黑场", "duration": 200000 }
+      },
+      "byDirector": {
+        "tarantino": { "name": "闪白", "duration": 100000 },
+        "kitano":    { "name": "溶解", "duration": 400000 },
+        "fincher":   { "name": "黑场", "duration": 200000 }
+      }
+    }
   }
 }