Move all nixos-modules out of qois subfolder

nixos-modules/backup-server/README.md

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+# Backup Server Module
+
+This backup module creates borg repositories for all the hosts configured with hosts.

nixos-modules/backup-server/default.nix

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+{
+  config,
+  lib,
+  options,
+  pkgs,
+  self,
+  ...
+}:
+
+let
+  cfg = config.qois.backup-server or { };
+in
+with lib;
+{
+  options.qois.backup-server = {
+    enable = mkEnableOption "Enable backup hosting";
+
+    backupStorageRoot = mkOption {
+      type = with types; nullOr str;
+      default = "/mnt/backup";
+      example = "/mnt/nas/backup";
+      description = "Path where backups are stored if this host is used as a backup target.";
+    };
+
+    hosts = options.qois.meta.hosts // {
+      default = config.qois.meta.hosts;
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.borgbackup.repos =
+      let
+        hasSshKey = hostName: cfg.hosts.${hostName}.sshKey != null;
+        mkRepo =
+          hostName:
+          (
+            let
+              name = "system-${hostName}";
+            in
+            {
+              inherit name;
+              value = {
+                path = "${cfg.backupStorageRoot}/${name}";
+                authorizedKeys = [ cfg.hosts.${hostName}.sshKey ];
+              };
+            }
+          );
+
+        hostsWithSshKeys = lib.filter hasSshKey (lib.attrNames cfg.hosts);
+      in
+      lib.listToAttrs (map mkRepo hostsWithSshKeys);
+  };
+}