Vps 91/prevent last scene deletion

[rgin216]

Issue

Deleting the only scene in a scenario allowed the scenario to end up with no scenes. This caused the authoring page and scenario info page to crash because both reasonably expect every scenario to have at least one scene.

Solution

Scenarios are now created with an initial Scene 1 on the backend, making the “at least one scene” invariant true from creation.

The scene delete API now rejects attempts to delete the last remaining scene with a 409 response. The reorder API also rejects empty sceneId arrays, so a scenario cannot be reordered into having no scenes.

The authoring UI now disables the delete action when only one scene remains and restores the scene list if a delete request fails.

Risk

Low. The main behaviour change is that scenarios can no longer be empty. Any existing code or tests that expected newly created scenarios to have zero scenes may need updating.

Existing database records with no scenes are not automatically backfilled by this PR.

Checklist

• Acceptance criteria met
• Wiki documentation is written and up to date
• Unit tests written and passing
• Integration tests written and passing
• Continuous integration build passing

[linear]

VPS-91

[leowla]

🎬🗑️

[harbassan]

awesome 😃