feat(infrastructure): Add database server templates

[nfebe]

PostgreSQL, MySQL, and MariaDB are still embedded in the agent instead of being available from the runtime template catalog. Add them as infrastructure templates that follow the current storage and network conventions.

Checklist

• Add PostgreSQL, MySQL, and MariaDB templates
• Store data in bind mounts inside each deployment directory
• Join the external database network without publishing ports by default
• Declare each template as infrastructure in its metadata
• Define the required credentials as deploy time inputs without usable defaults
• Document the new catalog entries

Related

• feat: Add support for infrastructure deployment ui#55
• feat: Add endpoints for provisioning infrastructure; mariadb, mysql, redis, cerbot nginx etc agent#107