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_synchronous_nodes.py

@@ -6,10 +6,10 @@ atomic operations, so those are covered here too.
 
 from concurrent.futures import ThreadPoolExecutor
 
-from app.flow.messages import DType, MessageSpec
-from app.flow.nodes import Node
-from app.flow.pipeline import Pipeline
-from app.flow.state import MemoryState
+from fluksio.flow.messages import DType, MessageSpec
+from fluksio.flow.nodes import Node
+from fluksio.flow.pipeline import Pipeline
+from fluksio.flow.state import MemoryState
 
 
 def spec(name: str) -> MessageSpec: