feat(metadata): overview falls back to Chinese variants only (MD-2)

- adapters: overviewLanguages() limits overview/tagline chain to Chinese
  variants (zh-CN→zh-SG→zh-TW→zh-HK); titles keep full chain; season
  supplement fetch skips languages useless for overview
- setting metadata.overviewNonChineseFallback (default off) + UI switch
- media detail shows "暂无简介" for empty entity/episode overview
- scripts/overview-recompute.ts: recompute stored non-Chinese overviews
  from kv_cache only (no TMDB requests), dry-run by default, --apply
- tests: chain unit tests, Heart Signal (103147) fixture, server
  recompute test, MP differential for episode overview
- runbook + CHANGELOG

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

apps/server/src/modules/file-manager/candidate.ts

@@ -5,7 +5,7 @@ import {
   createEntityFromTmdbCandidate,
   enqueueNewEntityRefresh,
 } from "../sources-identity/service/entity-from-candidate.ts";
-import { metadataLanguages } from "../sources-identity/service/metadata-rich.ts";
+import { metadataLanguages, metadataTextOptions } from "../sources-identity/service/metadata-rich.ts";
 import { actorOfP, FilesError, type FmDeps } from "./common.ts";
 import { audit } from "./ledger.ts";
 
@@ -24,7 +24,10 @@ export async function entityFromCandidate(
 ): Promise<FilesEntityFromCandidateResult> {
   try {
     const languages = await metadataLanguages(d.ctx);
-    const r = await createEntityFromTmdbCandidate(d.ctx.db, d.tmdb, ref, actorOf(principal), { languages });
+    const r = await createEntityFromTmdbCandidate(d.ctx.db, d.tmdb, ref, actorOf(principal), {
+      languages,
+      ...(await metadataTextOptions(d.ctx)),
+    });
     if (r.created) {
       await audit(d, principal, "entity_from_candidate", { ...ref, mediaEntityId: r.mediaEntityId });
       // 订阅建档 / 搜索建档 / 整理向导都走这里:建档即入队 full 刷新(每集信息、富字段)