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/test_metrics.py

@@ -10,9 +10,9 @@ from datetime import datetime, timezone
 
 from sqlmodel import Session, select
 
-from app.flow.events import EventBus
-from app.flow.metrics import DETAIL_CAP, RUN_STALE_S, MetricsCollector
-from app.models import EngineEvent, FlowRun, MetricBucket
+from fluksio.flow.events import EventBus
+from fluksio.flow.metrics import DETAIL_CAP, RUN_STALE_S, MetricsCollector
+from fluksio.models import EngineEvent, FlowRun, MetricBucket
 
 FLOW = "metrics-test"
 NODE = "metrics-test.calc"