Better backend and cache performance

[javuto]

Implementation of #562

Summary

• Replace per-node heartbeat writes with bounded bulk updates while preserving timestamps and preventing stale IP updates.
• Add race-safe query caching with a configurable two-minute default and post-commit invalidation.
• Cache console/file-explorer session checks and skip them when acceleration is disabled.
• Return retryable errors for query-dispatch failures and validate persisted TTL settings.

Validation

• Full Go suite and targeted race tests passed.
• PostgreSQL, MySQL, and Redis integration tests passed.
• OpenAPI consistency check passed.

Operational Notes

• Upgrade all API/TLS replicas for race-safe invalidation.
• Direct-database CLI submissions rely on cache expiry; API-mode submissions invalidate immediately.