fix: use U2-Net as default model (fast, 2s) with BiRefNet as opt-in

BiRefNet-Lite times out on first load (~60s+ for 973MB model).
U2-Net works in 2 seconds. Users can still select BiRefNet for
higher quality when they're willing to wait. Added timing hints
in model descriptions and increased timeout for BiRefNet models.

packages/ai/python/remove_bg.py

@@ -8,7 +8,7 @@ def main():
     output_path = sys.argv[2]
     settings = json.loads(sys.argv[3]) if len(sys.argv) > 3 else {}
 
-    model = settings.get("model", "birefnet-general-lite")
+    model = settings.get("model", "u2net")
     bg_color = settings.get("backgroundColor", "")
 
     try: