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.

backend/scripts/lint.sh

@@ -3,6 +3,6 @@
 set -e
 set -x
 
-mypy app
-ruff check app
-ruff format app --check
+mypy fluksio
+ruff check fluksio
+ruff format fluksio --check