Fix demo load: copy bundled clip to a temp file for Gradio cache

allowed_paths got past the 403, but Gradio 6 then refused to move the
bundled examples/pushback_demo.mp3 into its cache ('not uploaded by a
user'). Copy it to a per-request temp dir first so Gradio caches and
serves it like a normal upload, so 'Try the demo' loads and analyzes.

app.py

@@ -271,8 +271,16 @@ def finish_song(audio_path, total_seconds, vibe, remaster,
 
 
 def load_demo():
-    """Load the PUSHBACK demo clip into the uploader."""
-    return PUSHBACK_DEMO
+    """Load the PUSHBACK demo clip into the uploader.
+
+    Copy the bundled clip to a fresh temp file and hand THAT to the uploader.
+    Gradio 6 refuses to move a non-user-uploaded bundled file into its cache
+    ("…was not uploaded by a user"), which broke the demo on the Space; a file
+    in a per-request temp dir is one Gradio will cache and serve normally."""
+    import shutil
+    dst = os.path.join(tempfile.mkdtemp(), os.path.basename(PUSHBACK_DEMO))
+    shutil.copyfile(PUSHBACK_DEMO, dst)
+    return dst
 
 
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