nix: expose overlay and NixOS module for infrastructure deployment

- Fix default.nix to build from workspace (imphnen-backend/Cargo.toml)
- Add overlays.default adding pkgs.imphnen-backend
- Add nixosModules.backend via nixos-module.nix
- NixOS module defines systemd service on port 8081 with environmentFile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flake.nix

@@ -1,41 +1,47 @@
-{
-  description = "IMPHNEN Backend Service Nix Flake";
-
-  inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
-  };
-
-  outputs =
-    {
-      self,
-      nixpkgs,
-    }:
-    let
-      supportedSystems = [
-        "x86_64-linux"
-        "x86_64-darwin"
-        "aarch64-darwin"
-        "aarch64-linux"
-      ];
-      pkgsFor =
-        system:
-        import nixpkgs {
-          inherit system;
-          config = {
-            allowUnfree = true;
-          };
-        };
-      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
-    in
-    {
-      packages = forAllSystems (system: {
-        default = (pkgsFor system).callPackage ./default.nix { };
-      });
-      devShells = forAllSystems (system: {
-        default = (pkgsFor system).callPackage ./shell.nix { };
-      });
-      dockerImages = forAllSystems (system: {
-        tryOutApi = (pkgsFor system).callPackage ./docker.nix { };
-      });
-    };
-}
+{
+  description = "IMPHNEN Backend Service Nix Flake";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+    }:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+        "aarch64-linux"
+      ];
+      pkgsFor =
+        system:
+        import nixpkgs {
+          inherit system;
+          config.allowUnfree = true;
+        };
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+    in
+    {
+      packages = forAllSystems (system: {
+        default = (pkgsFor system).callPackage ./default.nix { };
+      });
+
+      overlays.default = final: _prev: {
+        imphnen-backend = final.callPackage ./default.nix { };
+      };
+
+      nixosModules.backend = ./nixos-module.nix;
+
+      devShells = forAllSystems (system: {
+        default = (pkgsFor system).callPackage ./shell.nix { };
+      });
+
+      dockerImages = forAllSystems (system: {
+        tryOutApi = (pkgsFor system).callPackage ./docker.nix { };
+      });
+    };
+}