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/scripts/tests-start.sh

@@ -2,6 +2,6 @@
 set -e
 set -x
 
-python app/tests_pre_start.py
+python -m fluksio.tests_pre_start
 
 bash scripts/test.sh "$@"