Finalize itis-sumo migration and Python 3.14 support

Scope

Reconcile the historical itis-sumo work from #537, #548, and #559 and make the backend/runtime migration production-ready.

Replace the legacy Dakota ownership boundary with the supported itis_sumo.api workflow layer.
Update Python metadata, CI, Docker images, and flaskapi/uv.lock for Python 3.14.
Verify the available Dakota/itis-sumo wheels and document the supported Python matrix.
Preserve backend API behavior and analytical workflows.

itis-sumo is the supported backend package and legacy orchestration is no longer on the runtime path.
Python 3.14 compatibility is demonstrated by lock, image, regular, analytical, and E2E checks.
Existing Flask/Dakota route contracts remain green.
Upgrade and compatibility constraints are documented.