FIX: delayed chat summary email (#31255)

Updates the chat summary email to account for:

- unread mentions in category channels (same as before)
- unread direct messages (now excluding threads)
- unread watched thread replies (for both channels and DM channels)

We have also reduced the window from 1 week down to 1 day for all 3
criteria. The DM unreads query is now properly selecting the first
unread message within the window (rather than the first message
regardless of read status).

plugins/chat/spec/mailers/user_notifications_spec.rb

@@ -579,4 +579,66 @@ describe UserNotifications do
       end
     end
   end
+
+  describe "in a direct message channel with threads" do
+    fab!(:message) do
+      Fabricate(:chat_message, chat_channel: direct_message, user: other, created_at: 2.days.ago)
+    end
+    fab!(:thread) { Fabricate(:chat_thread, channel: direct_message, original_message: message) }
+    fab!(:reply) { Fabricate(:chat_message, chat_channel: direct_message, thread:, user: other) }
+    let(:watching) { Chat::NotificationLevels.all[:watching] }
+
+    it "does not send a chat summary email for thread replies" do
+      no_chat_summary_email
+    end
+
+    describe "when the user is watching the thread" do
+      before do
+        Fabricate(:user_chat_thread_membership, user: user, thread:, notification_level: watching)
+      end
+
+      it "sends a chat summary email" do
+        chat_summary_email
+      end
+    end
+
+    describe "when the user has 2 watched threads" do
+      fab!(:message_2) do
+        Fabricate(
+          :chat_message,
+          chat_channel: direct_message_2,
+          user: another,
+          created_at: 2.days.ago,
+        )
+      end
+      fab!(:thread_2) do
+        Fabricate(:chat_thread, channel: direct_message_2, original_message: message_2)
+      end
+      fab!(:thread_2_reply) do
+        Fabricate(:chat_message, chat_channel: direct_message_2, thread: thread_2, user: another)
+      end
+
+      before do
+        Fabricate(:user_chat_thread_membership, user: user, thread:, notification_level: watching)
+        Fabricate(
+          :user_chat_thread_membership,
+          user: user,
+          thread: thread_2,
+          notification_level: watching,
+        )
+      end
+
+      it "sends a chat summary email" do
+        chat_summary_with_subject(:watched_threads, channel: direct_message.title(user), count: 1)
+      end
+    end
+
+    describe "when another user is watching a thread" do
+      before { thread.membership_for(other).update!(notification_level: watching) }
+
+      it "does not send current user a chat summary email" do
+        no_chat_summary_email
+      end
+    end
+  end
 end