feat: 生命周期广播事件与链式钩子设施（ADR-006）

- contracts:补齐生命周期广播事件(订阅、下载、整理结束、刮削、字幕、作品关注 / 元数据 / 季集结构、站点可达性)
  的类型与 payload schema,LIFECYCLE_EVENTS 清单与 lifecycleKey 幂等键;HOOK_POINTS 链式钩子挂点;
  FEATURES 功能清单与 SLOT_POSITIONS 前端插槽位置
- outbox 中继支持一条事件路由给多个订阅方(其余订阅方用 <键>#<任务名>);生命周期事件允许多模块订阅,
  没有订阅方时也被中继认领并镜像到 SSE
- 服务端 HookRegistry:按注册顺序执行、rewrite / intercept 两类、单钩子超时与异常隔离、无注册零开销;
  ServerModule 增加 manifest 与 registerHooks,bootstrap 装配 ctx.hooks
- ADR-006 与 server README「如何写一个功能模块」

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Cxwgnn2pbzQ7rXv3rq7fei

packages/contracts/src/index.ts

@@ -14,6 +14,8 @@ export * from "./download-board.ts";
 export * from "./episode-mapping.ts";
 export * from "./events.ts";
 export * from "./execution.ts";
+export * from "./features.ts";
+export * from "./hooks.ts";
 export * from "./identity.ts";
 export * from "./instances.ts";
 export * from "./library-versions.ts";