feat(transfer): 新增 TransferOverwriteCheck 事件支持插件介入覆盖判断

允许插件在覆盖模式判断前提供目标文件的真实大小或直接给出覆盖决策，
解决 .strm 等本地大小不准的场景下 size 模式失效的问题。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/schemas/types.py

@@ -156,6 +156,8 @@ class ChainEventType(Enum):
     TransferRename = "transfer.rename"
     # 整理拦截
     TransferIntercept = "transfer.intercept"
+    # 整理覆盖检查
+    TransferOverwriteCheck = "transfer.overwrite.check"
     # 资源选择
     ResourceSelection = "resource.selection"
     # 资源下载