enable and document using manage.py in online environment

allows e.g. making users from cli

panel/README.md

@@ -55,3 +55,9 @@ The Fediversity Panel is a web service for managing Fediversity deployments with
   ```
 
 - Use the [Django Debug Toolbar](https://github.com/django-commons/django-debug-toolbar) for troubleshooting in the browser
+
+When deployed, `manage` may be accessed like:
+
+```sh
+sudo DATABASE_URL=sqlite:////var/lib/panel/db.sqlite3 manage
+```

panel/src/panel/settings.py

@@ -25,24 +25,9 @@ BASE_DIR = Path(__file__).resolve().parent.parent
 # Quick-start development settings - unsuitable for production
 # See https://docs.djangoproject.com/en/4.2/howto/deployment/checklist/
 
-def get_secret(name: str, encoding: str = "utf-8") -> str:
-    # In the NixOS deployment, this variable is set by `systemd` via `LoadCredential`
-    # https://systemd.io/CREDENTIALS/
-    credentials_dir = env.get("CREDENTIALS_DIRECTORY")
-
-    if credentials_dir is None:
-        raise RuntimeError("No credentials directory available.")
-
-    try:
-        with open(f"{credentials_dir}/{name}", encoding=encoding) as f:
-            secret = f.read().removesuffix("\n")
-    except FileNotFoundError:
-        raise RuntimeError(f"No secret named {name} found in {credentials_dir}.")
-
-    return secret
-
+# mandatory for the systemd service, but not set (or needed) for manage.py.
 # SECURITY WARNING: keep the secret key used in production secret!
-SECRET_KEY = get_secret("SECRET_KEY")
+SECRET_KEY = env.get("CREDENTIALS_DIRECTORY")
 
 # SECURITY WARNING: don't run with debug turned on in production!
 DEBUG = True
@@ -189,11 +174,11 @@ if user_settings_file is not None:
 # non-Django application settings
 
 # TODO(@fricklerhandwerk):
-#     The correct thing to do here would be using a helper function such as with `get_secret()` that will catch the exception and explain what's wrong and where to put the right values.
+#     The correct thing to do here would be using a helper function that will catch the exception and explain what's wrong and where to put the right values.
 #     Replacing the `USER_SETTINGS_FILE` mechanism following the comment there would probably be a good thing.
 
 # a dir of nix supporting experimental feature `configurable-impure-env`.
-nix_bin=env['NIX_BIN']
+nix_bin=env.get('NIX_BIN')
 # path of the root flake to trigger nixops from, see #94.
 # to deploy this should be specified, for dev just use a relative path.
-repo_dir = env["REPO_DIR"]
+repo_dir = env.get("REPO_DIR")