Ship the bundled PUSHBACK demo clip (via Git LFS)

The demo audio matched '*.mp3' in .gitignore, so it was never committed
or deployed — 'Try the demo' had no file on the Space, which surfaced as a
cascade of 403 / cache-move / split errors that all traced back to a
missing file. Track *.mp3 with LFS (HF rejects plain binaries) and add the
clip so the demo loads. Pairs with the load_demo temp-copy + allowed_paths.

.gitattributes

@@ -33,3 +33,4 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text
 *.zip filter=lfs diff=lfs merge=lfs -text
 *.zst filter=lfs diff=lfs merge=lfs -text
 *tfevents* filter=lfs diff=lfs merge=lfs -text
+*.mp3 filter=lfs diff=lfs merge=lfs -text

examples/pushback_demo.mp3

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:cf8ede66ebff48a629701519547b8931149321ba8521f63f86125b0259b20cc5
+size 473427