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/Application/Services/StudioService.cs

@@ -230,6 +230,31 @@ public class StudioService(StudioDbContext db)
         return await GetProjectAsync(id, userId);
     }
 
+    /// <summary>
+    /// Update individual scene-input values by content key (the studio editor's live
+    /// edits). Writes to saved_scene_contents.value so the render binder uses them.
+    /// </summary>
+    public async Task<int> UpdateSceneContentsAsync(Guid projectId, Guid userId, List<UpdateContentItem> items)
+    {
+        var owns = await db.SavedProjects.AnyAsync(x => x.Id == projectId && x.UserId == userId);
+        if (!owns) throw new KeyNotFoundException($"Project {projectId} not found");
+
+        var updated = 0;
+        foreach (var item in items)
+        {
+            if (string.IsNullOrWhiteSpace(item.Key)) continue;
+            // ExecuteSqlInterpolated maps C# null → typed SQL NULL (raw DBNull params throw).
+            updated += await db.Database.ExecuteSqlInterpolatedAsync($@"
+                UPDATE studio.saved_scene_contents c
+                SET value = {item.Value}, value_file_id = COALESCE({item.ValueFileId}, c.value_file_id), updated_at = now()
+                FROM studio.saved_scenes s
+                WHERE c.saved_scene_id = s.id AND s.saved_project_id = {projectId} AND c.key = {item.Key}");
+        }
+        await db.Database.ExecuteSqlInterpolatedAsync(
+            $"UPDATE studio.saved_projects SET last_edit_date = now(), updated_at = now() WHERE id = {projectId}");
+        return updated;
+    }
+
     public async Task DeleteProjectAsync(Guid id, Guid userId)
     {
         var project = await db.SavedProjects