Fix: restore @spaces.GPU decorator required by ZeroGPU

Decorator was accidentally dropped during model refactor.
ZeroGPU fails at startup with "No @spaces.GPU function detected"
without it.

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

image_generator.py

@@ -167,6 +167,12 @@ def _generate(animals: list[str], places: list[str]):
 # Public API — two versions depending on environment
 # ---------------------------------------------------------------------------
 
-def generate_reward_image(animals: list[str], places: list[str]):
-    """Generate reward image — works locally (MPS/CPU) and on HF Spaces (persistent GPU)."""
-    return _generate(animals, places)
+if IS_HF_SPACE:
+    @spaces.GPU(duration=60)
+    def generate_reward_image(animals: list[str], places: list[str]):
+        """Generate reward image on HF Spaces ZeroGPU."""
+        return _generate(animals, places)
+else:
+    def generate_reward_image(animals: list[str], places: list[str]):
+        """Generate reward image locally (MPS/CPU)."""
+        return _generate(animals, places)