perf(api): cache total wallets metric

[raymondjacobson]

Summary

• add a tiny per-server TTL cache for /v1/metrics/total_wallets
• keep the cache bounded to 16 entries with a 5 minute TTL
• add handler coverage for the cached response path

Evidence

• prod read-only pg_stat_statements showed the total-wallets aggregate query at roughly 750k calls, 943k total seconds, and ~1.26s mean time since stats reset
• the endpoint returns a coarse aggregate count, so a short TTL removes repeated count scans while accepting only brief staleness

Test Plan

• go test ./api -run 'TestMetricsTotalWallets' -count=1\n- git diff --check