* fix: update env template for systemd compatibility

Adjust env template to ensure compatibility with systemd's EnvironmentFile parsing, which only recognizes line-starting comment symbols.

* Refactor SMTP and Rocket settings in .env.template

- Simplify the SMTP_SECURITY and SMTP_PORT options by providing a list of choices and default values
- Clarify the ROCKET_PORT default value depending on the environment (Docker or not)