Fix: Don't cache nil in default_role_id to prevent bootstrap issues

- Only cache non-nil role_id values to allow retry after role creation
- Prevents processes from being permanently stuck with nil if first call
  happens before the 'Mitglied' role exists
- Update documentation to explain bootstrap safety mechanism

lib/accounts/user.ex

@@ -445,6 +445,13 @@ defmodule Mv.Accounts.User do
   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.
 
+  ## Bootstrap Safety
+
+  Only non-nil values are cached. If the role doesn't exist yet (e.g., before seeds run),
+  `nil` is not cached, allowing subsequent calls to retry after the role is created.
+  This prevents bootstrap issues where a process would be permanently stuck with `nil`
+  if the first call happens before the role exists.
+
   ## Performance Note
 
   This function makes one database query per process (cached in process dictionary).
@@ -464,6 +471,9 @@ defmodule Mv.Accounts.User do
   @spec default_role_id() :: Ecto.UUID.t() | nil
   def default_role_id do
     # Cache in process dictionary to avoid repeated queries
+    # IMPORTANT: Only cache non-nil values to avoid bootstrap issues.
+    # If the role doesn't exist yet (e.g., before seeds run), we don't cache nil
+    # so that subsequent calls can retry after the role is created.
     case Process.get({__MODULE__, :default_role_id}) do
       nil ->
         role_id =
@@ -472,7 +482,8 @@ defmodule Mv.Accounts.User do
             _ -> nil
           end
 
-        Process.put({__MODULE__, :default_role_id}, role_id)
+        # Only cache non-nil values to allow retry if role is created later
+        if role_id, do: Process.put({__MODULE__, :default_role_id}, role_id)
         role_id
 
       cached_role_id ->