Add sendspin-cli package with Python dependencies

Integrate sendspin-cli as a custom package using python3.withPackages.
Packages aiosendspin from GitHub since it's only available in
nixpkgs-unstable. Includes all required dependencies: aiohttp, av,
numpy, qrcode, readchar, rich, sounddevice, and native libraries
(portaudio, ffmpeg).

Adds uv2nix flake inputs for future migration when sendspin-cli
adds a uv.lock file.

machines/zix790prors/configuration.nix

@@ -9,7 +9,7 @@ with lib;
 {
   imports = [
     ./hardware-configuration.nix
-    #./virtual-surround.nix
+    ./virtual-surround.nix
   ];
 
   roles = {