feat(api): load config using shared helpers

- add django settings module documentation
- use default for previously required fields

BREAKING CHANGE: The API command line interface require the
configuration file to be present. The default configuration file path is
`/etc/airtime/airtime.conf`

api/setup.py

@@ -43,6 +43,7 @@ setup(
         "dev": [
             "model_bakery",
             "psycopg2-binary",
+            f"libretime-shared @ file://localhost/{here.parent / 'shared'}#egg=libretime_shared",
         ],
     },
 )