Add NOT NULL constraint to users.role_id and optimize default_role_id

- Add database-level NOT NULL constraint for users.role_id
- Update SystemActor tests to verify NOT NULL constraint enforcement
- Add process dictionary caching for default_role_id/0 to reduce DB queries

lib/accounts/user.ex

@@ -442,6 +442,15 @@ defmodule Mv.Accounts.User do
   It fetches the "Mitglied" role from the database without authorization checks
   (safe during user creation bootstrap phase).
 
+  The result is cached in the process dictionary to avoid repeated database queries
+  during high-volume user creation. The cache is invalidated on application restart.
+
+  ## Performance Note
+
+  This function makes one database query per process (cached in process dictionary).
+  For very high-volume scenarios, consider using a fixed UUID from Application config
+  instead of querying the database.
+
   ## Returns
 
   - UUID of the "Mitglied" role if it exists
@@ -454,9 +463,20 @@ defmodule Mv.Accounts.User do
   """
   @spec default_role_id() :: Ecto.UUID.t() | nil
   def default_role_id do
-    case Mv.Authorization.Role.get_mitglied_role() do
-      {:ok, %Mv.Authorization.Role{id: role_id}} -> role_id
-      _ -> nil
+    # Cache in process dictionary to avoid repeated queries
+    case Process.get({__MODULE__, :default_role_id}) do
+      nil ->
+        role_id =
+          case Mv.Authorization.Role.get_mitglied_role() do
+            {:ok, %Mv.Authorization.Role{id: id}} -> id
+            _ -> nil
+          end
+
+        Process.put({__MODULE__, :default_role_id}, role_id)
+        role_id
+
+      cached_role_id ->
+        cached_role_id
     end
   end
 end