feat(observability): pass GRAFANA_DB_PASSWORD to archive-backend

Flyway runs inside the backend container at startup; V68's
${grafanaDbPassword} placeholder is resolved from this env var.

Refs #651.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -163,6 +163,9 @@ services:
       SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/${POSTGRES_DB}
       SPRING_DATASOURCE_USERNAME: ${POSTGRES_USER}
       SPRING_DATASOURCE_PASSWORD: ${POSTGRES_PASSWORD}
+      # Consumed by Flyway V68 via the ${grafanaDbPassword} placeholder to set
+      # the read-only grafana_reader role's password.
+      GRAFANA_DB_PASSWORD: ${GRAFANA_DB_PASSWORD}
       S3_ENDPOINT: http://minio:9000
       S3_ACCESS_KEY: ${MINIO_ROOT_USER}
       S3_SECRET_KEY: ${MINIO_ROOT_PASSWORD}