auth: create a proper selfprivacy-api token via auth module

- selfprivacy-api NixOS module can use selfprivacy.auth.clients option
  to configure its own client
- when "selfprivacy-api" OAuth ID name is used, read-write token is
  created and idm_admins membership is set

flake.lock

@@ -62,11 +62,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1744640029,
-        "narHash": "sha256-Rqy+HhW7weEfAc5rPmxusewuo/69sWVXlQOL2a3Y9ZU=",
-        "ref": "inex/add-oauth",
-        "rev": "5393642f896d5fa4b9b7be215d72714554259de6",
-        "revCount": 1749,
+        "lastModified": 1745270427,
+        "narHash": "sha256-EHbn9AgWTmIuRwAo7Y+sULHNN+/vN0r8h2JbTqmYxZc=",
+        "ref": "use-auth-nixos-module",
+        "rev": "582d11c1fbea36d6fdbd0a64f782ca8f8c6a1338",
+        "revCount": 1750,
         "type": "git",
         "url": "https://git.selfprivacy.org/SelfPrivacy/selfprivacy-rest-api.git"
       },