feat(observability): wire obs-grafana to archive-db and inject GRAFANA_DB_PASSWORD

obs-grafana now joins archiv-net so it can resolve archive-db:5432 for the
PO Overview dashboard's PostgreSQL datasource, and receives GRAFANA_DB_PASSWORD
so provisioning can interpolate it into the datasource config.

Refs #651.

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

docker-compose.observability.yml

@@ -147,6 +147,9 @@ services:
       GF_SECURITY_ADMIN_PASSWORD: ${GRAFANA_ADMIN_PASSWORD:-changeme}
       GF_USERS_ALLOW_SIGN_UP: "false"
       GF_SERVER_ROOT_URL: ${GF_SERVER_ROOT_URL:-http://localhost:3003}
+      # Read-only password for the grafana_reader PostgreSQL role; interpolated
+      # into the provisioned PostgreSQL datasource (see datasources.yml).
+      GRAFANA_DB_PASSWORD: ${GRAFANA_DB_PASSWORD}
     volumes:
       - grafana_data:/var/lib/grafana
       - ./infra/observability/grafana/provisioning:/etc/grafana/provisioning:ro
@@ -165,6 +168,7 @@ services:
         condition: service_healthy
     networks:
       - obs-net
+      - archiv-net   # PO Overview dashboard queries archive-db via the grafana_reader role
 
   # --- Error Tracking: GlitchTip ---