feat(db): download_task 增加缺席记录与代数字段（迁移 0067）

missing_since / missing_count 记录 qB 里连续查不到的起点与轮数；generation 记录 removed 行复活次数，用于完成事件幂等键换代。contracts DownloadTask 同步为可选字段。

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

packages/db/src/types.ts

@@ -719,6 +719,9 @@ export interface DownloadTaskTable {
   updated_at: GeneratedTimestamptz;
   media_entity_id: string | null;
   import_plan_id: string | null;
+  missing_since: Timestamptz | null;
+  missing_count: Generated<number>;
+  generation: Generated<number>;
 }
 
 export interface DownloaderInstanceTable {