feat(library): download history lists adopted tasks and organize records

- Migration 0053: download_task.media_entity_id (FK, set null on delete),
  backfilled from plan slot / source release / latest organize record.
- Submitted and adopted tasks carry the entity; entity merge follows it.
- GET /library/{id}: downloads also matched by dt.media_entity_id (adopted
  takeover tasks), plus organizationRecords (incl. manual path organizes);
  library list / inventory counts use it too.
- Web detail: "接管" marker and a list of records without a download task.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/execution.ts

@@ -189,6 +189,8 @@ export const DownloadTask = EntityBase.extend({
   infohashV2: z.string().nullable(),
   acquisitionPlanId: Id.nullable(),
   sourceReleaseId: Id.nullable(),
+  /** 关联作品:计划 / 站点资源 / 接管任务按文件名识别 / 手动指定 / 手动整理后回填(QB-1) */
+  mediaEntityId: Id.nullable().optional(),
   name: z.string(),
   savePath: z.string().nullable(),
   contentPath: z.string().nullable(),