v5.3.1 - Security update of the bundled database image

Security update. No code changes, no database migrations and no configuration
changes. Updating is recommended for all installations.

Security

• Updated the bundled PostgreSQL/pgvector image in docker-compose.yml to a
  current release with the latest security fixes.

Upgrade notes

• Pull the new image and recreate the database container:
  docker compose pull && docker compose up -d. Your data stays in the
  mounted data directory (./data/postgres by default).