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/libretime_api/settings/__init__.py

@@ -0,0 +1,46 @@
+# pylint: disable=unused-import
+from os import getenv
+
+from ._internal import (
+    AUTH_PASSWORD_VALIDATORS,
+    AUTH_USER_MODEL,
+    DEBUG,
+    INSTALLED_APPS,
+    MIDDLEWARE,
+    REST_FRAMEWORK,
+    ROOT_URLCONF,
+    TEMPLATES,
+    TEST_RUNNER,
+    WSGI_APPLICATION,
+    setup_logger,
+)
+from ._schema import Config
+
+API_VERSION = "2.0.0"
+
+LIBRETIME_LOG_FILEPATH = getenv("LIBRETIME_LOG_FILEPATH")
+LIBRETIME_CONFIG_FILEPATH = getenv("LIBRETIME_CONFIG_FILEPATH")
+
+CONFIG = Config(filepath=LIBRETIME_CONFIG_FILEPATH)
+
+SECRET_KEY = CONFIG.general.api_key
+ALLOWED_HOSTS = ["*"]
+
+DATABASES = {
+    "default": {
+        "ENGINE": "django.db.backends.postgresql",
+        "HOST": CONFIG.database.host,
+        "PORT": CONFIG.database.port,
+        "NAME": CONFIG.database.name,
+        "USER": CONFIG.database.user,
+        "PASSWORD": CONFIG.database.password,
+    }
+}
+
+LANGUAGE_CODE = "en-us"
+TIME_ZONE = "UTC"
+USE_I18N = True
+USE_L10N = True
+USE_TZ = True
+
+LOGGING = setup_logger(LIBRETIME_LOG_FILEPATH)