chore: remove leftover Sora references after platform removal

PR #1463 removed the Sora platform, but some references survived:

- README/README_CN/README_JA kept the 'Sora status (temporarily
  unavailable)' sections and gateway.sora_* docs that the removal PR
  never touched.
- deploy/config.example.yaml still documented ~130 lines of sora_*
  gateway keys, the top-level sora: direct-client/storage block, and
  token_refresh.sync_linked_sora_accounts - none of which map to any
  field in the config structs anymore.
- The OIDC login PR (02a66a01c, branched off pre-removal main and
  merged 4 days after #1463) re-added the dead
  PublicSettings.SoraClientEnabled field, which no code ever sets.
- A release sync (748a84d87) re-introduced sora i18n keys that the
  later i18n split (d9e514f98) faithfully carried into
  locales/{zh,en}/admin/{overview,settings}.ts. No component references
  any of these keys.

This drops all of the above. Pure deletions, no behavior change.

README_JA.md

@@ -550,12 +550,6 @@ default:
   rate_multiplier: 1.0
 ```
 
-### Sora ステータス（一時的に利用不可）
-
-> ⚠️ Sora 関連の機能は、上流統合およびメディア配信の技術的問題により一時的に利用できません。
-> 現時点では本番環境で Sora に依存しないでください。
-> 既存の `gateway.sora_*` 設定キーは予約されていますが、これらの問題が解決されるまで有効にならない場合があります。
-
 `config.yaml` では追加のセキュリティ関連オプションも利用できます:
 
 - `cors.allowed_origins` - CORS 許可リスト