Add NixOS module and deployment guide for lidarr-mb-gap

- Introduced a new NixOS module for the lidarr-mb-gap service, allowing users to configure and manage the report generation process through NixOS.
- Added a comprehensive deployment guide in `nixos/DEPLOYMENT.md`, detailing setup instructions, configuration options, and troubleshooting tips for deploying the service on NixOS and serving reports via Caddy.
- Updated `flake.nix` to export the new NixOS module.
- Enhanced the report generation scripts to support customizable output paths for generated reports.

flake.nix

@@ -7,7 +7,17 @@
   };
 
   outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
+    let
+      # NixOS module output (not system-specific)
+      # The module accepts a package option, which can be set from the flake's packages
+      nixosModules = {
+        lidarr-mb-gap = import ./nixos/lidarr-mb-gap.nix;
+      };
+    in
+    {
+      # Export NixOS modules
+      nixosModules = nixosModules;
+    } // flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs { inherit system; };
         lib = pkgs.lib;