use strict context boundaries

lib/memex/contexts/context.ex

@@ -27,7 +27,7 @@ defmodule Memex.Contexts.Context do
     field :tags_string, :string, virtual: true
     field :visibility, Ecto.Enum, values: [:public, :private, :unlisted]
 
-    belongs_to :user, User
+    field :user_id, :binary_id
 
     timestamps()
   end
@@ -38,7 +38,6 @@ defmodule Memex.Contexts.Context do
           tags: [String.t()] | nil,
           tags_string: String.t() | nil,
           visibility: :public | :private | :unlisted,
-          user: User.t() | Ecto.Association.NotLoaded.t(),
           user_id: User.id(),
           inserted_at: NaiveDateTime.t(),
           updated_at: NaiveDateTime.t()