Postgres connection env vars

[lovasoa]

Add support for standard PostgreSQL environment variables to configure database connections.

This allows SQLPage to automatically configure PostgreSQL connections using PGHOST, PGUSER, PGPASSWORD, etc., when no database_url is explicitly set. It also enables using PGPASSWORD if a configured database_url lacks a password and database_password is not explicitly provided, ensuring environment variables are respected while explicit configuration takes precedence.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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