feat: wire Laravel config for Beacon-managed PostgreSQL

[ibourgeois]

Context

As a developer choosing Beacon-managed PostgreSQL, I need the Laravel application configuration to be wired to that database automatically so the deployed app can connect without manual YAML edits.

Acceptance Criteria

• generate application database environment configuration for Beacon-managed PostgreSQL
• keep connection host, port, database, username, and password values aligned with the PostgreSQL deployment option
• integrate database credentials with Beacon secret handling choices
• preserve compatibility when PostgreSQL is not managed by Beacon
• add tests for the generated database configuration

Notes

This issue should build on the PostgreSQL deployment option rather than duplicating chart work.