Refactor flake.nix and enhance project structure

- Updated `flake.nix` to define a new Python application `lidarr-mb-gap` for identifying missing albums on MusicBrainz.
- Improved development shell environment by including a Python environment with necessary packages.
- Added new source files: `__init__.py`, `html_report.py`, and `main.py` to implement core functionality and HTML report generation.
- Introduced `pyproject.toml` for better package management and project metadata.
- Enhanced user instructions in the shell hook for running the application.

flake.nix

@@ -10,30 +10,46 @@
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs { inherit system; };
-        pythonEnv = pkgs.python3.withPackages (ps: with ps; [
-          requests
-          python-dotenv
-        ]);
+        lib = pkgs.lib;
+        lidarr-mb-gap = pkgs.python3Packages.buildPythonApplication {
+          pname = "lidarr-mb-gap";
+          version = "1.0.0";
+          src = lib.cleanSource ./src;
+          format = "pyproject";
+          nativeBuildInputs = with pkgs.python3Packages; [
+            setuptools
+          ];
+          propagatedBuildInputs = with pkgs.python3Packages; [
+            requests
+            python-dotenv
+          ];
+        };
       in
       {
         devShells.default = pkgs.mkShell {
-          buildInputs = [ 
-            pythonEnv
+          buildInputs = [
+            (pkgs.python3.withPackages (ps: with ps; [
+              requests
+              python-dotenv
+            ]))
             pkgs.black
           ];
           shellHook = ''
             echo "Python environment ready!"
-            echo "Run: python main.py"
-            echo "Format code with: black main.py"
+            echo "Run: python src/main.py"
+            echo "Format code with: black src/"
           '';
         };
 
-        packages.default = pkgs.writeShellApplication {
-          name = "lidarr-musicbrainz";
-          runtimeInputs = [ pythonEnv ];
-          text = ''
-            python ${./main.py} "$@"
-          '';
+        packages.default = lidarr-mb-gap;
+        packages.lidarr-mb-gap = lidarr-mb-gap;
+        apps.default = {
+          type = "app";
+          program = "${lidarr-mb-gap}/bin/lidarr-mb-gap";
+        };
+        apps.lidarr-mb-gap = {
+          type = "app";
+          program = "${lidarr-mb-gap}/bin/lidarr-mb-gap";
         };
       }
     );