feat(studio): per-scene loop plays on hover (scene.demo end-to-end)

Wires the per-scene loop video all the way to the scene card:
- studio-svc: SavedSceneResponse now includes Demo (was stored + copied but never
  serialized); MapSceneResponse passes s.Demo.
- Scene type gains image?/demo?; parseScene reads them from the loaded scene data.
- SceneThumbnailBlock shows scene.image as the still and plays scene.demo (muted,
  looped) on hover, resetting on mouse-leave.

Existing projects backfilled (saved_scenes.image/demo from content.scenes). Both
services rebuilt + deployed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/studio/FlatRender.StudioSvc/Application/Services/StudioService.cs

@@ -473,7 +473,7 @@ public class StudioService(StudioDbContext db)
     );
 
     private static SavedSceneResponse MapSceneResponse(SavedScene s) => new(
-        s.Id, s.OriginalSceneId, s.Key, s.Title, s.Image, s.SceneType,
+        s.Id, s.OriginalSceneId, s.Key, s.Title, s.Image, s.Demo, s.SceneType,
         s.Sort, s.SceneLengthSec, s.MinDurationSec, s.MaxDurationSec,
         s.OverlapAtEndSec, s.CanHandleDuration, s.ManualColorSelection, s.SelectedColorPresetId,
         s.Contents.Select(MapContentResponse).ToList(),

services/studio/FlatRender.StudioSvc/Models/Responses/Responses.cs

@@ -62,6 +62,7 @@ public record SavedSceneResponse(
     string Key,
     string? Title,
     string? Image,
+    string? Demo,
     string SceneType,
     int Sort,
     decimal SceneLengthSec,