fix: correct FFmpeg probe request parameter name

The /probe endpoint expects 'gcs_uri' but we were sending 'source_gcs_uri'.
Fixed to match the ProbeRequest model in ffmpeg_http_service.py.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/video_renderer.py

@@ -174,7 +174,7 @@ class VideoRendererService:
 
         response = client.post(
             f"{service_url}/probe",
-            json={"source_gcs_uri": gcs_uri},
+            json={"gcs_uri": gcs_uri},  # Matches ProbeRequest model
             headers={"Authorization": f"Bearer {auth_token}"}
         )
         response.raise_for_status()