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/Controllers/StudioController.cs

@@ -55,6 +55,11 @@ public class StudioController(StudioService svc) : ControllerBase
     public async Task<IActionResult> SaveScenes(Guid id, [FromBody] List<SaveSceneRequest> scenes) =>
         Ok(await svc.SaveScenesAsync(id, UserId, scenes));
 
+    /// <summary>Update individual scene-input values by content key (live studio edits).</summary>
+    [HttpPatch("{id:guid}/contents")]
+    public async Task<IActionResult> UpdateContents(Guid id, [FromBody] UpdateContentsRequest req) =>
+        Ok(new { updated = await svc.UpdateSceneContentsAsync(id, UserId, req.Items ?? new List<UpdateContentItem>()) });
+
     /// <summary>Internal endpoint: get project for render service (no user-ownership check).</summary>
     [HttpGet("{id:guid}/render-payload")]
     [Authorize(Roles = "Service")]