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.

src/__init__.py

@@ -0,0 +1,2 @@
+"""Lidarr to MusicBrainz Missing Albums Finder"""
+