infra: init backend api

nix/infra/backend.nix

@@ -0,0 +1,39 @@
+{
+  lib,
+  config,
+  self,
+  ...
+}:
+
+let
+  user = "alisa";
+in
+{
+  systemd.services.alisa-backend = {
+    enable = true;
+    after = [ "network.target" ];
+    wantedBy = [ "multi-user.target" ];
+    description = "Alisa Backend API Server";
+    environment = {
+      DATABASE_URL = "postgres://${user}@localhost:${config.services.postgresql.settings.port}/${user}";
+      TOKEN_SECRET = "secret";
+    };
+    serviceConfig = {
+      Restart = "always";
+      DynamicUser = true;
+      User = user;
+      ExecStart = lib.getExe self.legacyPackages.${config.nixpkgs.hostPlatform.system}.backend;
+    };
+  };
+
+  services.postgresql = {
+    enable = true;
+    ensureDatabases = [ user ];
+    ensureUsers = [
+      {
+        name = user;
+        ensureDBOwnership = true;
+      }
+    ];
+  };
+}