Adds a read-only datasource pointing at archive-db using the grafana_reader
role (provisioned by Flyway V68). The password is interpolated from the
GRAFANA_DB_PASSWORD env var passed to obs-grafana, and the connection is
locked to editable: false so the credential cannot be inspected via the UI.
sslmode=disable is intentional: traffic stays inside archiv-net.
Refs #651.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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