Improve event dashboard loading and caching

[psam21]

Summary

• Add a 30-second in-memory cache for the 2026+ event breakdown and warm it asynchronously at handler startup.
• Replace the year-discovery plus per-year queries with one grouped event aggregation query.
• Add a validated /api/events endpoint for cache hydration and refresh status.
• Show a clear loading state instead of a misleading zero-valued events table while the cache is cold.
• Keep the last cached dashboard count visible during background refreshes.
• Add regression coverage for year filtering and monthly, row, and grand totals.

Validation

• go test ./...
• go test -race ./...
• go vet ./...
• go build -o /tmp/ns-relay ./cmd
• node --check relay/web/static/script.js
• git diff --check