refactor: remove unused fields parameter from fuzzy_search API

The fields parameter was accepted but never used in the :search action.
Simplify API to only accept the query parameter.
Update @doc to reflect the actual functionality.

lib/membership/member.ex

@@ -481,7 +481,6 @@ defmodule Mv.Membership.Member do
     - `query` - Ash.Query.t() to apply search to
     - `opts` - Keyword list or map with search options:
       - `:query` or `"query"` - Search string
-      - `:fields` or `"fields"` - Optional field restrictions
 
   ## Returns
     - Modified Ash.Query.t() with search filters applied
@@ -502,13 +501,7 @@ defmodule Mv.Membership.Member do
     if String.trim(q) == "" do
       query
     else
-      args =
-        case opts[:fields] || opts["fields"] do
-          nil -> %{query: q}
-          fields -> %{query: q, fields: fields}
-        end
-
-      Ash.Query.for_read(query, :search, args)
+      Ash.Query.for_read(query, :search, %{query: q})
     end
   end