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/api/routes/test_observability.py

@@ -3,8 +3,8 @@ from datetime import datetime, timedelta, timezone
 from fastapi.testclient import TestClient
 from sqlmodel import Session
 
-from app.core.config import settings
-from app.models import EngineEvent, FlowRun, MetricBucket
+from fluksio.core.config import settings
+from fluksio.models import EngineEvent, FlowRun, MetricBucket
 
 PREFIX = f"{settings.API_V1_STR}/observability"
 FLOW = "observability-test"
@@ -90,7 +90,7 @@ def test_a_flow_that_cannot_run_makes_the_summary_degraded(
     knows is what this reports — and reporting it is the whole point: a flow
     with a dependency loop cannot run, and the screen used to say "ok".
     """
-    from app.flow.pipeline import ValidationIssue
+    from fluksio.flow.pipeline import ValidationIssue
 
     controller = client.app.state.flow_controller
     before = controller.issues