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.md

@@ -550,12 +550,6 @@ default:
   rate_multiplier: 1.0
 ```
 
-### Sora Status (Temporarily Unavailable)
-
-> ⚠️ Sora-related features are temporarily unavailable due to technical issues in upstream integration and media delivery.
-> Please do not rely on Sora in production at this time.
-> Existing `gateway.sora_*` configuration keys are reserved and may not take effect until these issues are resolved.
-
 Additional security-related options are available in `config.yaml`:
 
 - `cors.allowed_origins` for CORS allowlist