Draft: chore: update Grafana to 12.3.1 and PostgreSQL to 18
- Update Grafana from 12.0.2 to 12.3.1 (latest stable)
- Update PostgreSQL from 15 to 18 in docker-compose and Helm
- Update Grafana datasource postgresVersion from 1500 to 1800
PostgreSQL 18 brings up to 3x I/O performance improvements and better index utilization. pgwatch v3 supports PostgreSQL 14-18 with version-aware queries that automatically adapt.