Add Nix package, NixOS module, and tests for bambuddy

- packages.bambuddy: FastAPI backend + prebuilt React frontend assembled
  into the app-root layout upstream expects, behind a single `bambuddy`
  launcher (uvicorn --loop asyncio, ffmpeg on PATH, XDG state defaults).
  Pins upstream maziggy/bambuddy v1.2.5.7.
- nixosModules.default: services.bambuddy with host/port/dataDir,
  openFirewall, environment + environmentFile, extraPackages,
  DynamicUser + StateDirectory, systemd hardening, and
  CAP_NET_BIND_SERVICE for the virtual-printer FTPS/RTSP ports.
- checks.bambuddy-import: backend import + admin CLI smoke test.
- checks.bambuddy-service: two-node NixOS VM test covering the health
  endpoint, UI/static assets/manifest, SQLite + logs in the
  StateDirectory, journal startup check, cross-node reachability with
  openFirewall, custom host/port/env node, and loopback-only binding.
- overlay, run app, devShell, README, .gitignore.

flake.nix

@@ -0,0 +1,98 @@
+{
+  description = "Bambuddy — self-hosted command center for Bambu Lab 3D printers (package + NixOS module)";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+
+      pkgsFor = system: nixpkgs.legacyPackages.${system};
+    in
+    {
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = pkgsFor system;
+        in
+        rec {
+          bambuddy = pkgs.callPackage ./pkgs/bambuddy { };
+          default = bambuddy;
+        }
+      );
+
+      overlays.default = final: prev: {
+        bambuddy = self.packages.${final.stdenv.hostPlatform.system}.bambuddy;
+      };
+
+      nixosModules = rec {
+        # Wires the module against this flake's own package as the default,
+        # so `services.bambuddy.enable = true;` works out of the box.
+        bambuddy =
+          { lib, pkgs, ... }:
+          {
+            imports = [ ./nixos/module.nix ];
+            services.bambuddy.package = lib.mkOptionDefault self.packages.${pkgs.stdenv.hostPlatform.system}.bambuddy;
+          };
+        default = bambuddy;
+      };
+
+      checks = forAllSystems (
+        system:
+        let
+          pkgs = pkgsFor system;
+          bambuddy = self.packages.${system}.bambuddy;
+        in
+        {
+          bambuddy-import = pkgs.callPackage ./tests/import-check.nix { inherit bambuddy; };
+        }
+        // nixpkgs.lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
+          bambuddy-service = pkgs.callPackage ./tests/service-test.nix {
+            inherit bambuddy;
+            nixosModule = self.nixosModules.default;
+          };
+        }
+      );
+
+      apps = forAllSystems (
+        system:
+        let
+          bambuddy = self.packages.${system}.bambuddy;
+        in
+        {
+          default = {
+            type = "app";
+            program = "${bambuddy}/bin/bambuddy";
+            meta.description = "Run the Bambuddy server";
+          };
+        }
+      );
+
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = pkgsFor system;
+          bambuddy = self.packages.${system}.bambuddy;
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [
+              bambuddy.passthru.pythonEnv
+              pkgs.nodejs
+              pkgs.ffmpeg
+              pkgs.nixfmt
+            ];
+          };
+        }
+      );
+    };
+}