Expose Arena ELO sync in feature flags (#3821)

Adds ARENA_ELO_SYNC_ENABLED to the Dashboard Feature Flags registry (DB-overridable),
routes Arena ELO startup/status checks through the shared feature-flag resolver while
preserving the existing env fallback, and refreshes env docs (adds the missing
STREAM_READINESS_TIMEOUT_MS example) so env/doc sync stays green.

Integrated into release/v3.8.25.

Co-authored-by: R.D. <rogerproself@gmail.com>

src/shared/utils/featureFlags.ts

@@ -75,3 +75,7 @@ export function isCcCompatibleProviderEnabled(): boolean {
 export function isModelCatalogNamesEnabled(): boolean {
   return isFeatureFlagEnabled("MODEL_CATALOG_INCLUDE_NAMES");
 }
+
+export function isArenaEloSyncEnabled(): boolean {
+  return isFeatureFlagEnabled("ARENA_ELO_SYNC_ENABLED");
+}