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/flow/test_messages.py

@@ -2,7 +2,7 @@ import json
 
 import pytest
 
-from app.flow.messages import DType, MessageSpec, flow_of, qualify
+from fluksio.flow.messages import DType, MessageSpec, flow_of, qualify
 
 
 def test_port_defaults_to_last_name_segment():