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/web/app/components/settings/metadata.tsx

@@ -51,7 +51,14 @@ const TMDB_CARD: CardSpec = {
       label: "语言回退链",
       kind: "text",
       placeholder: DEFAULT_LANGUAGES.join(", "),
-      hint: "标题、简介和每集信息按顺序取第一个有值的语言；中文名取链中第一个中文值。逗号分隔，如 zh-CN, zh-SG",
+      hint: "标题和每集信息按顺序取第一个有值的语言，简介默认只取链里的中文；中文名取链中第一个中文值。逗号分隔，如 zh-CN, zh-SG",
+      wide: true,
+    },
+    {
+      name: "metadata.overviewNonChineseFallback",
+      label: "简介回退到非中文",
+      kind: "switch",
+      hint: "没有中文简介时，按回退链取英文等语言。关闭时留空，标题不受影响",
       wide: true,
     },
     {
@@ -68,6 +75,7 @@ const TMDB_CARD: CardSpec = {
     "metadata.tmdbBaseUrl": s.metadata.tmdbBaseUrl ?? "",
     "metadata.languages": (s.metadata.languages ?? DEFAULT_LANGUAGES).join(", "),
     "metadata.includeSpecials": s.metadata.includeSpecials !== false,
+    "metadata.overviewNonChineseFallback": s.metadata.overviewNonChineseFallback === true,
   }),
   toPatch: (v) => {
     const key = secretPatch(v["metadata.tmdbApiKey"]);
@@ -76,6 +84,7 @@ const TMDB_CARD: CardSpec = {
         tmdbBaseUrl: str(v["metadata.tmdbBaseUrl"]) || null,
         languages: parseLanguages(str(v["metadata.languages"])),
         includeSpecials: v["metadata.includeSpecials"] === true,
+        overviewNonChineseFallback: v["metadata.overviewNonChineseFallback"] === true,
         ...(key !== undefined ? { tmdbApiKey: key } : {}),
       },
     };