feat: scaffold Nix flake with dotnet-sdk_9, typst, and nodejs

- devShells.default provides dotnetCorePackages.sdk_9_0, typst, nodejs
- packages.default uses buildDotnetModule with net9.0 target via callPackage
- Stub plugin/ project with minimal .csproj
- .gitignore with .NET/Nix build artifacts

flake.nix

@@ -0,0 +1,72 @@
+{
+  description = "Jellyfin Lessons — a Jellyfin plugin";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      forAllSystems =
+        fn: nixpkgs.lib.genAttrs nixpkgs.lib.systems.flakeExposed fn;
+    in
+    {
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [
+              pkgs.dotnetCorePackages.sdk_9_0
+              pkgs.typst
+              pkgs.nodejs
+            ];
+
+            shellHook = ''
+              echo "jellyfin-lessons dev shell"
+              echo "  dotnet: $(dotnet --version)"
+              echo "  typst:  $(typst --version)"
+              echo "  node:   $(node --version)"
+            '';
+          };
+        }
+      );
+
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.callPackage (
+            {
+              buildDotnetModule,
+              dotnetCorePackages,
+              lib,
+            }:
+            buildDotnetModule {
+              pname = "jellyfin-lessons";
+              version = "0.1.0";
+
+              src = self;
+
+              projectFile = "plugin/jellyfin-lessons.csproj";
+              nugetDeps = ./deps.json;
+
+              dotnet-sdk = dotnetCorePackages.sdk_9_0;
+              dotnet-runtime = dotnetCorePackages.aspnetcore_9_0;
+
+              meta = {
+                description = "Jellyfin Lessons plugin";
+                license = lib.licenses.mit;
+                platforms = lib.platforms.all;
+              };
+            }
+          ) { };
+        }
+      );
+    };
+}