fix: lower validator cache default to 250K entries
the validator key cache was set to 500K entries but the pod OOMs at
~450K — eviction never triggers. indigo uses 5M with LRU+TTL eviction;
rsky uses 262K. 250K matches rsky and fits within the 3 GiB pod limit.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>