feat: add PostgreSQL infrastructure for App2

- Add PostgreSQL role for installing and configuring PostgreSQL 17
- Add postgres_exporter role for PostgreSQL metrics collection
- Add deploy-postgres-app2.yml playbook for deployment
- Configure test database 'testdb' with user 'testuser'
- Set up postgres_exporter user for monitoring
- Include firewall configuration for PostgreSQL (5432) and postgres_exporter (9187)
- Follow existing role structure pattern from node_exporter

roles/postgres_exporter/templates/postgres_exporter.service.j2

@@ -0,0 +1,16 @@
+[Unit]
+Description=Postgres Exporter
+After=network.target postgresql.service
+Wants=postgresql.service
+
+[Service]
+Type=simple
+User=postgres_exporter
+Group=postgres_exporter
+Environment=DATA_SOURCE_NAME="{{ postgres_exporter_data_source_name }}"
+ExecStart=/usr/local/bin/postgres_exporter --web.listen-address=:{{ postgres_exporter_port }}
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target