[SECURITY][Medium] Weak default sample database passwords in .env

## Summary
Sample env file used extremely weak default DB credentials.

## Severity
Medium

## Affected Files
.env

## Recommended Remediation
Replace defaults with strong placeholders requiring explicit secure values.

## Patch Branch
codex/sec-env-weak-passwords

## Patch Commit
63eb9bf

## Fork Branch URL
https://github.com/lhy8888/cmdb-security-fork/tree/codex/sec-env-weak-passwords