Fix: preprocess image on remote TRELLIS.2 Space before calling image_to_3d

image_to_3d uses preprocess_image=False — it expects the image to already
have background removed and be cropped to the object bounding box. In the
UI flow this is done by the image_prompt.upload() event handler before
generation runs. When calling via gradio_client we bypass that handler,
sending a raw RGB image that causes the model to raise RuntimeError.

Fix: add a Step 0 that calls /preprocess_image on the remote Space first.
This runs background removal (BRIA-RMBG via the Space's own rmbg_client)
and crops to bbox, producing the RGBA image image_to_3d expects.
Falls back to raw image if preprocessing endpoint is unavailable.

src/stages/stage1_generate.py

@@ -84,10 +84,28 @@ def generate_trellis(
 
     client = _get_trellis_client()
 
+    from gradio_client import handle_file
+
+    # Step 0: preprocess on the remote Space (background removal + bbox crop).
+    # image_to_3d expects a preprocessed RGBA image — in the UI flow this is
+    # done by the image_prompt.upload() event before generation is triggered.
+    # When calling via API we must do it explicitly or the model crashes.
+    try:
+        preprocessed_path = client.predict(
+            handle_file(image_path),
+            api_name="/preprocess_image",
+        )
+        gen_image = handle_file(
+            preprocessed_path if isinstance(preprocessed_path, str) else image_path
+        )
+    except Exception:
+        # If remote preprocessing is unavailable, send the raw image and hope
+        # the image already has a transparent background.
+        gen_image = handle_file(image_path)
+
     try:
-        from gradio_client import handle_file
         state, _html = client.predict(
-            image=handle_file(image_path),
+            image=gen_image,
             seed=int(seed),
             resolution=preset["resolution"],
             ss_guidance_strength=7.5,
@@ -105,7 +123,11 @@ def generate_trellis(
             api_name="/image_to_3d",
         )
     except Exception as e:
-        return f"❌ TRELLIS.2 generation failed:\n```\n{type(e).__name__}: {e}\n```"
+        return (
+            f"❌ TRELLIS.2 generation failed:\n```\n{type(e).__name__}: {e}\n```\n\n"
+            f"Tip: make sure your image has a clean subject on a white/transparent "
+            f"background for best results."
+        )
 
     try:
         hp_src, _ = client.predict(