Rename `Inbox` model to `InboxEntry`

app/models/inbox_entry.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class Inbox < ApplicationRecord
+class InboxEntry < ApplicationRecord
   belongs_to :user, touch: :inbox_updated_at
   belongs_to :question
 

spec/factories/inbox_entry.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 FactoryBot.define do
-  factory :inbox do
+  factory :inbox_entry do
     question { FactoryBot.build(:question) }
     new { true }
   end

spec/models/inbox_entry_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-describe Inbox, type: :model do
+describe InboxEntry, type: :model do
   describe "associations" do
     it { should belong_to(:user) }
     it { should belong_to(:question) }
@@ -13,7 +13,7 @@ describe Inbox, type: :model do
     let(:question) { FactoryBot.create(:question, author_is_anonymous: true) }
 
     it "does not fail if the user wants to delete their account" do
-      Inbox.create(user:, question:)
+      InboxEntry.create(user:, question:)
 
       # this deletes the User record and enqueues the deletion of all
       # associated records in sidekiq