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]>

packages/contracts/src/settings.ts

@@ -246,6 +246,11 @@ export const MetadataSettings = z.object({
     .min(1)
     .max(8)
     .default(["zh-CN", "zh-SG", "zh-TW", "zh-HK", "en-US"]),
+  /**
+   * MD-2:简介 / 标语是否允许回退到非中文。缺省 false = 只按链里的中文变体(及排在第一个中文之前的语言)取,
+   * 取不到留空;标题不受影响。链里没有中文时不起作用
+   */
+  overviewNonChineseFallback: z.boolean().default(false),
   /**
    * DH-1 季 0(特别篇):true = full 刷新与元数据回刷在 TMDB 有季 0 时一并拉取每集信息并建集单元(对照 MP);
    * 季 0 不计入正片季总集数与订阅完成判定
@@ -649,6 +654,7 @@ export const DEFAULT_SETTINGS: GlobalSettings = {
     tmdbApiKeySecretRef: null,
     tmdbBaseUrl: null,
     languages: ["zh-CN", "zh-SG", "zh-TW", "zh-HK", "en-US"],
+    overviewNonChineseFallback: false,
     includeSpecials: true,
   },
   sites: {
@@ -954,6 +960,7 @@ export const SETTINGS_SECTIONS: readonly SettingsSectionDef[] = [
       "metadata.tmdbApiKeySecretRef",
       "metadata.tmdbBaseUrl",
       "metadata.languages",
+      "metadata.overviewNonChineseFallback",
       "metadata.includeSpecials",
       "cache.tmdbMaxSize",
       "identity.eagerTmdbLookup",