Fix Grafana dashboard import validation

[variablenix]

Summary

• convert the dashboard from an instance-bound v2 API resource to Grafana's portable Classic import model
• map Prometheus through DS_PROMETHEUS during import
• remove server-managed generation and timestamp state that can fail Import/Save validation
• preserve all nine panels, five dashboard variables, layout, queries, and descriptions
• add regression checks for the portable data-source input and dashboard identity

Validation

• jq empty grafana/gobot-dashboard.json
• go test ./...
• git diff --check