Make CLP_DB_USER environment variable optional with default value

[coderabbitai]

Description

Make the CLP_DB_USER environment variable optional with a default value of "clp-user" in the Docker Compose configuration.

Context

This was identified during review of PR #1655 (comment: #1655 (comment)).

Currently, CLP_DB_USER is a required environment variable. Making it optional with a sensible default would improve the developer experience for local deployments.

Requested by

@junhaoliao