Avoid sqlite fallback during Shipit builds

[theduke]

Summary

• keep local development using SQLite when no database env vars are set
• use MySQL placeholder settings during Shipit build-time Django commands so collectstatic does not import Python's sqlite3 module
• preserve the existing MySQL env-var path for deployed apps

Context

The backend integration template canary failed for django-wasmer-starter in wasmerio/backend run 27819941199, job 82339241719. Remote build failed in python manage.py collectstatic --noinput with ModuleNotFoundError: No module named '_sqlite3' because the template fell back to SQLite while Shipit had detected the template as a MySQL Django app.

Validation

• python -m py_compile mysite/settings.py
• uvx shipit-cli --regenerate --temp-shipit --skip-prepare --no-wasmer --no-docker . was attempted locally; it reaches dependency installation but cannot build mysqlclient on this host because local MySQL/MariaDB pkg-config development files are not installed. The CI remote build image has those dependencies and reached collectstatic, which is the failure addressed here.