feat: add PostgreSQL admin user with superuser access (#367)

Adds a `mu` user with superuser privileges for managing all databases
on glyph.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: stackptr

hosts/glyph/services/db.nix

@@ -14,6 +14,10 @@
     };
     ensureDatabases = ["atticd" "grafana" "open-webui" "pocketid"];
     ensureUsers = [
+      {
+        name = "mu";
+        ensureClauses.superuser = true;
+      }
       {
         name = "atticd";
         ensureDBOwnership = true;