Add logging for fail-open email uniqueness validations

Log warnings when query errors occur in email uniqueness checks
to improve visibility of data integrity issues

lib/mv/membership/member/validations/email_not_used_by_other_user.ex

@@ -63,7 +63,13 @@ defmodule Mv.Membership.Member.Validations.EmailNotUsedByOtherUser do
       {:ok, _} ->
         {:error, field: :email, message: "is already used by another user", value: email}
 
-      {:error, _} ->
+      {:error, reason} ->
+        require Logger
+
+        Logger.warning(
+          "Email uniqueness validation query failed for member email '#{email}': #{inspect(reason)}. Allowing operation to proceed (fail-open)."
+        )
+
         :ok
     end
   end