fix: correct default DB_PORT from 3306 to 5432 for postgresql dialect (fixes #642)

The default DB_DIALECT is 'postgresql' but DB_PORT defaulted to 3306 (MySQL's
port), causing connection failures for users who deployed without an explicit
DB_PORT in their .env. Changed the default to 5432 and updated documentation
in .env.example, README.md, and README-EN.md to be consistent.

Co-Authored-By: Octopus <liyuan851277048@icloud.com>