Update Postgres images to 18

[techulus-agent]

Summary

• update bundled production Postgres image to postgres:18
• update local dev Postgres image to postgres:18-alpine

Validation

• docker compose -f compose.dev.yml config --quiet
• docker compose -f deployment/compose.postgres.yml config --quiet with a temporary .env
• docker manifest inspect postgres:18
• docker manifest inspect postgres:18-alpine