perf: remove custom field search from user-linking autocomplete

Custom field LIKE queries on JSONB are expensive (no index).
User linking only needs name/email search for autocomplete.
Custom fields are still searchable via main member search (uses FTS index).
Remove unnecessary credo:disable as function complexity is now acceptable.

lib/membership/member.ex

@@ -579,9 +579,9 @@ defmodule Mv.Membership.Member do
   # - Empty user_email ("") → email == "" is always false → only fuzzy search matches
   # - This allows a single filter expression instead of duplicating fuzzy search logic
   #
-  # Cyclomatic complexity is unavoidable here: PostgreSQL fuzzy search requires
-  # multiple OR conditions for good search quality (FTS + trigram similarity + substring)
-  # credo:disable-for-next-line Credo.Check.Refactor.CyclomaticComplexity
+  # Note: Custom field search is intentionally excluded from linking to optimize
+  # autocomplete performance. Custom fields are still searchable via the main
+  # member search which uses the indexed search_vector.
   defp apply_linking_filters(query, user_email, search_query) do
     has_search = search_query && String.trim(search_query) != ""
     # Use empty string instead of nil to simplify filter logic
@@ -591,11 +591,8 @@ defmodule Mv.Membership.Member do
       # Search query provided: return email-match OR fuzzy-search candidates
       trimmed_search = String.trim(search_query)
 
-      pat = "%" <> trimmed_search <> "%"
-
-      # Build search filters using modular functions for maintainability
+      # Build search filters - excluding custom_field_filter for performance
       fts_match = build_fts_filter(trimmed_search)
-      custom_field_match = build_custom_field_filter(pat)
       fuzzy_match = build_fuzzy_filter(trimmed_search, @default_similarity_threshold)
       email_substring_match = expr(contains(email, ^trimmed_search))
 
@@ -606,7 +603,6 @@ defmodule Mv.Membership.Member do
           # If email is "", this is always false and search filters take over
           email == ^trimmed_email or
             ^fts_match or
-            ^custom_field_match or
             ^fuzzy_match or
             ^email_substring_match
         )