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.

backend/internal/handler/dto/public_settings_injection_schema_test.go

@@ -29,8 +29,6 @@ func TestPublicSettingsInjectionPayload_SchemaDoesNotDrift(t *testing.T) {
 
 	// Fields that legitimately live only on the DTO. Keep tiny; document each.
 	dtoOnlyFields := map[string]string{
-		// sora_client_enabled is an upstream-only field the fork does not surface.
-		"sora_client_enabled": "upstream-only field, not used on this fork",
 		// force_email_on_third_party_signup lives on the DTO but is not injected via SSR.
 		"force_email_on_third_party_signup": "auth-source default, not a feature flag",
 	}

backend/internal/handler/dto/settings.go

@@ -397,7 +397,6 @@ type PublicSettings struct {
 	OIDCOAuthProviderName               string                   `json:"oidc_oauth_provider_name"`
 	GitHubOAuthEnabled                  bool                     `json:"github_oauth_enabled"`
 	GoogleOAuthEnabled                  bool                     `json:"google_oauth_enabled"`
-	SoraClientEnabled                   bool                     `json:"sora_client_enabled"`
 	BackendModeEnabled                  bool                     `json:"backend_mode_enabled"`
 	PaymentEnabled                      bool                     `json:"payment_enabled"`
 	Version                             string                   `json:"version"`