[pack] Grafana 12485 PostgreSQL Exporter

[shmsr]

Parent

GitHub sub-issue of https://github.com/elastic/obs-integration-team/issues/1200 (Curated Grafana dashboard packs).

Dashboard

• grafana.com: https://grafana.com/grafana/dashboards/12485
• gnetId: 12485
• Pack dir: grafana_12485_postgresql_exporter

Done when

• Pack is in registry.yaml and auto-fires.
• fidelity_manifest.yaml has no UNKNOWN.
• Live validate + render audit: no render_error on PERFECT/APPROXIMATE panels.
• Operator-visible notes in docs/sources/grafana.md if output changed.
• PR merged.

Engine bugs found while curating this dashboard go to https://github.com/elastic/obs-integration-team/issues/1201, not here.