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_senders.py

@@ -2,9 +2,9 @@
 
 import asyncio
 
-from app.flow.messages import DType, MessageSpec
-from app.flow.nodes import MqttNode
-from app.flow.nodes.http import close_shared_client, shared_client
+from fluksio.flow.messages import DType, MessageSpec
+from fluksio.flow.nodes import MqttNode
+from fluksio.flow.nodes.http import close_shared_client, shared_client
 
 
 def test_http_senders_share_one_pooled_client():