feat(studio B1): persist input edits to content elements (render-binding foundation)

Studio edits previously went only to edit_state; the render binds saved_scene_contents,
so edits never reached the MP4. Add studio-svc PATCH /v1/saved-projects/{id}/contents
(update saved_scene_contents.value/value_file_id by content key, ExecuteSqlInterpolated
for null-safe params) + Next /api/projects/[id]/contents route + persistence hook pushes
edited values (from bridged c-<key> layers) alongside the scene_data save. Verified
text persists incl. UTF-8 Persian (9 chars/17 bytes).

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

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

@@ -123,3 +123,8 @@ public record SavedProjectListRequest(
     string? Q = null,
     string? Type = null
 );
+
+/// <summary>Lightweight update of individual scene-input values — the studio editor
+/// writes the user's edits here (by content key) so the render binder picks them up.</summary>
+public record UpdateContentsRequest(List<UpdateContentItem> Items);
+public record UpdateContentItem(string Key, string? Value, Guid? ValueFileId);