Rename the import package app to fluksio

A wheel whose top-level module is `app` collides with anything else in a
user's venv, so the package that is about to be published takes the name
it is published under. Only the Python package moves; the repo, the
Docker WORKDIR and the compose project keep theirs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/api/routes/test_private.py

@@ -2,8 +2,8 @@ import pytest
 from fastapi.testclient import TestClient
 from sqlmodel import Session, select
 
-from app.core.config import settings
-from app.models import User
+from fluksio.core.config import settings
+from fluksio.models import User
 
 
 def test_create_user(client: TestClient, db: Session) -> None: