Stringify landmark dict keys before pose response

orjson (the JSON renderer Gradio uses) rejects non-string dict keys
with TypeError: Dict key must be str. detect_landmarks returns
{int: {...}} so the /jiggle/pose response failed to serialize even
though the GPU work succeeded. Convert keys to str in the response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -242,10 +242,12 @@ def _register_jiggle_routes(app: FastAPI) -> None:
         except Exception as e:
             raise HTTPException(status_code=500, detail=str(e))
 
+        # orjson (Gradio's response renderer) rejects non-string dict keys —
+        # detect_landmarks returns {int: {...}} so we stringify before returning.
         return {
             "transforms": transforms,
-            "tpose_landmarks": tpose_lm,
-            "target_landmarks": target_lm,
+            "tpose_landmarks": {str(k): v for k, v in tpose_lm.items()},
+            "target_landmarks": {str(k): v for k, v in target_lm.items()},
         }
 
     # Promote our routes ahead of Gradio's catch-all (GET /{path:path} SPA route)