Optimize jump to unread to reduce flicker on large backlog load.

tests/test_api.py

@@ -1005,6 +1005,23 @@ class TestReadStateEndpoints:
         assert result["last_read_ats"][f"channel-{chan_key}"] == 1000
         assert result["last_read_ats"][f"contact-{contact_key}"] == 1000
 
+        # The oldest unread message per conversation, used by the client to place
+        # the unread divider without paging back through history.
+        first_chan = await MessageRepository.get_by_content(
+            msg_type="CHAN",
+            conversation_key=chan_key,
+            text="Bob: hello",
+            sender_timestamp=1001,
+        )
+        assert result["first_unread_ids"][f"channel-{chan_key}"] == first_chan.id
+        first_dm = await MessageRepository.get_by_content(
+            msg_type="PRIV",
+            conversation_key=contact_key,
+            text="hi @[TeStUsEr] there",
+            sender_timestamp=1005,
+        )
+        assert result["first_unread_ids"][f"contact-{contact_key}"] == first_dm.id
+
     @pytest.mark.asyncio
     async def test_get_unreads_no_name_skips_mentions(self, test_db):
         """Unreads without a radio name returns counts but no mention flags."""