feat(src/djinit/templates/config/settings): add DATABASE_URL support via dj_database_url

Add conditional import of `dj_database_url` and a directive comment for `use_database_url` to enable automatic database configuration when the `use_database_url` flag is set, improving flexibility of the production settings template.

- New Features: Introduce support for reading `DATABASE_URL` to configure Django databases automatically.
- Enhancements: Conditional import improves template modularity and reduces unnecessary dependencies.
- Bug Fixes: None.

src/djinit/templates/config/settings/production.py-tpl

@@ -1,7 +1,9 @@
 """generated with djinit"""
 
 from .base import *
+# @IF use_database_url
 import dj_database_url
+# @ENDIF
 
 DEBUG = False
 SECRET_KEY = env('SECRET_KEY')