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.

packages/sendspin-cli/default.nix

@@ -0,0 +1,91 @@
+{ pkgs
+, lib
+, fetchFromGitHub
+, uv2nix ? null
+, pyproject-nix ? null
+, pyproject-build-systems ? null
+}:
+
+# Simple package build
+# Note: uv2nix would be ideal but requires uv.lock which sendspin-cli doesn't have yet
+let
+  # Package aiosendspin from GitHub since it's only in nixpkgs-unstable
+  aiosendspin = pkgs.python312Packages.buildPythonPackage rec {
+    pname = "aiosendspin";
+    version = "1.2.0";
+    pyproject = true;
+
+    src = fetchFromGitHub {
+      owner = "Sendspin";
+      repo = "aiosendspin";
+      rev = version;
+      sha256 = "sha256-3vTEfXeFqouPswRKST/9U7yg9ah7J9m2KAMoxaBZNR0=";
+    };
+
+    build-system = with pkgs.python312Packages; [
+      hatchling
+      setuptools
+    ];
+
+    dependencies = with pkgs.python312Packages; [
+      aiohttp
+      av
+      mashumaro
+      orjson
+      pillow
+      zeroconf
+    ];
+
+    pythonImportsCheck = [ "aiosendspin" ];
+
+    meta = {
+      description = "Async Python implementation of the Sendspin Protocol";
+      homepage = "https://github.com/Sendspin-Protocol/aiosendspin";
+      license = lib.licenses.asl20;
+    };
+  };
+
+  python = pkgs.python312.withPackages (ps: with ps; [
+    # Core dependencies from pyproject.toml
+    aiosendspin
+    av
+    numpy
+    qrcode
+    readchar
+    rich
+    sounddevice
+    setuptools
+  ]);
+in
+pkgs.stdenv.mkDerivation rec {
+  pname = "sendspin-cli";
+  version = "0.0.0";
+
+  src = fetchFromGitHub {
+    owner = "Sendspin";
+    repo = "sendspin-cli";
+    rev = "main";
+    sha256 = "sha256-z8ieaDHv4C6WNLpPGybhcfB+E6Jj/rCc7zSRpL6vdk0=";
+  };
+
+  buildInputs = [ python pkgs.portaudio pkgs.ffmpeg ];
+
+  installPhase = ''
+    mkdir -p $out/bin $out/lib
+    cp -r sendspin $out/lib/
+    cat > $out/bin/sendspin <<EOF
+#!/bin/sh
+export PYTHONPATH="$out/lib:\$PYTHONPATH"
+export LD_LIBRARY_PATH="${pkgs.portaudio}/lib:${pkgs.ffmpeg}/lib:\$LD_LIBRARY_PATH"
+exec ${python}/bin/python3 -m sendspin.cli "\$@"
+EOF
+    chmod +x $out/bin/sendspin
+  '';
+
+  meta = {
+    description = "Synchronized audio player for Sendspin servers";
+    homepage = "https://github.com/Sendspin/sendspin-cli";
+    license = lib.licenses.asl20;
+    platforms = lib.platforms.linux;
+  };
+}