refactor: remove redundant ilike filter in build_substring_filter

contains(city, ^query) already produces ILIKE '%query%'
ilike(city, ^pattern) with pattern="%query%" is identical

lib/membership/member.ex

@@ -515,14 +515,13 @@ defmodule Mv.Membership.Member do
   # Builds substring search filter for structured fields
   # Note: contains/2 uses ILIKE '%value%' which is not index-optimized
   # Performance: Good for small datasets, may be slow on large tables
-  defp build_substring_filter(query, pattern) do
+  defp build_substring_filter(query, _pattern) do
     expr(
       contains(postal_code, ^query) or
         contains(house_number, ^query) or
         contains(phone_number, ^query) or
         contains(email, ^query) or
-        contains(city, ^query) or
-        ilike(city, ^pattern)
+        contains(city, ^query)
     )
   end